run_wml_tests: Don't append to the provided path if it's not a directory

This allows passing an explicit path on Mac (instead of using the automatic -p xcode option), since the binary is not named "wesnoth" on that platform.

run_wml_tests

@@ -127,6 +127,7 @@ class WesnothRunner:
                 path = os.path.split(os.path.realpath(sys.argv[0]))[0]
             else:
                 path = options.path
+            if os.path.isdir(path):
             path += "/wesnoth"
             if options.debug_bin:
                 path += "-debug"