Improves the WML unit test script's error reporting.

This commit:
* Adds the new `bt` (batch timeout) option, to limit the maximum time batched unit tests can take.  Otherwise, if for whatever reason they take longer than 10 minutes, travis will error the job due to not receiving any output for too long.
* Writes the test output to files rather than trying to get it from the stderr, and then output's the log from the failed test.  Currently if a test unexpectedly times out, there is nothing printed indicating the error or which test encountered the problem (in case of batched tests).

run_wml_tests

@@ -58,6 +58,12 @@ class TestListParser:
             test_list.append(t)
         return test_list
 
+def get_output_filename(args):
+    for i,arg in enumerate(args):
+        if arg == "-u":
+            return "test-output-"+args[i+1]
+    raise RuntimeError("No -u option found!")
+
 def run_with_rerun_for_sdl_video(args, timeout):
     """A wrapper for subprocess.run with a workaround for the issue of travis+18.04
     intermittently failing to initialise SDL.
@@ -68,9 +74,14 @@ def run_with_rerun_for_sdl_video(args, timeout):
     while sdl_retries < 10:
         # For compatibility with Ubuntu 16.04 LTS, this has to run on Python3.5,
         # so the capture_output argument is not available.
-        res = subprocess.run(args, timeout=timeout, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-        if res.stderr.find(b"Could not initialize SDL_video") == -1:
-            return res
+        filename = get_output_filename(args)
+        res = subprocess.run(args, timeout=timeout, stdout=open(filename, "w"), stderr=subprocess.STDOUT)
+        retry = False
+        with open(filename, "r") as output:
+            if "Could not initialize SDL_video" in output.read():
+                retry = True
+        if not retry:
+        	  return res
         sdl_retries += 1
         print("Could not initialise SDL_video error, attempt", sdl_retries)
 
@@ -105,6 +116,7 @@ class WesnothRunner:
         if options.additional_arg is not None:
             self.common_args.extend(options.additional_arg)
         self.timeout = options.timeout
+        self.batch_timeout = options.batch_timeout
         if self.verbose > 1:
             print("Options that will be used for all Wesnoth instances:", repr(self.common_args))
 
@@ -124,8 +136,10 @@ class WesnothRunner:
         if self.timeout == 0:
             timeout = None
         else:
-            # could add a separate option for batch timeouts, but for now this is enough
-            timeout = int (self.timeout * (1 + 0.2 * len(test_list)))
+            if len(test_list) == 1:
+                timeout = self.timeout
+            else:
+                timeout = self.batch_timeout
         if len(test_list) == 1:
             print("Running test", test_list[0].name)
         else:
@@ -136,13 +150,10 @@ class WesnothRunner:
         try:
             res = run_with_rerun_for_sdl_video(args, timeout)
         except subprocess.TimeoutExpired as e:
-            # Using subprocess.run we can't access the captured output,
-            # for now just create a result so that this can be handled
-            # by the same code path as other statuses.
-            res = subprocess.CompletedProcess(args, UnitTestResult.TIMEOUT.value,
-                stderr = b"Timed out (killed by Python timeout implementation)")
+            print("Timed out (killed by Python timeout implementation)")
+            res = subprocess.CompletedProcess(args, UnitTestResult.TIMEOUT.value)
         if self.verbose > 1:
-            print("Result:", res.returncode, "\n", res.stderr)
+            print("Result:", res.returncode)
         if res.returncode < 0:
             print("Wesnoth exited because of signal", -res.returncode)
             if options.backtrace:
@@ -152,7 +163,9 @@ class WesnothRunner:
                 subprocess.run(gdb_args, timeout=240)
             raise UnexpectedTestStatusException()
         if res.returncode != expected_result.value:
-            print(str(res.stderr, encoding="utf-8", errors="backslashreplace"))
+            with open(get_output_filename(args), "r") as output:
+                for line in output.readlines():
+                	print(line)
             print("Failure, Wesnoth returned", res.returncode, "but we expected", expected_result.value)
             raise UnexpectedTestStatusException()
 
@@ -180,6 +193,8 @@ if __name__ == '__main__':
         help="Additional arguments to go to wesnoth. For options that start with a hyphen, '--add_argument --data-dir' will give an error, use '--add_argument=--data-dir' instead.")
     ap.add_argument("-t", "--timeout", type=int, default=10,
         help="New timer value to use, instead of 10s as default. The value 0 means no timer, and also skips tests that expect timeout.")
+    ap.add_argument("-bt", "--batch-timeout", type=int, default=300,
+        help="New timer value to use for batched tests, instead of 300s as default.")
     ap.add_argument("-s", "--no-strict", dest="strict_mode", action="store_false",
         help="Disable strict mode. By default, we run wesnoth with the option --log-strict=warning to ensure errors result in a failed test.")
     ap.add_argument("-d", "--debug_bin", action="store_true",