headless-browser: Bump test timeout from 5s to 15s

We're seeing occasional timeouts on the very first test when running
on CI. Let's try giving it more time to see if it goes away. :^)

Userland/Utilities/headless-browser.cpp

@@ -180,7 +180,7 @@ enum class TestMode {
     Text,
 };
 
-static ErrorOr<String> run_one_test(HeadlessWebContentView& view, StringView input_path, StringView expectation_path, TestMode mode, int timeout_in_milliseconds = 5000)
+static ErrorOr<String> run_one_test(HeadlessWebContentView& view, StringView input_path, StringView expectation_path, TestMode mode, int timeout_in_milliseconds = 15000)
 {
     Core::EventLoop loop;
     bool did_timeout = false;