Make gTestRunnerHeadless into a constant outside of tests (#8306)

Co-authored-by: Hedara <hedara90@gmail.com>

include/test_runner.h

@@ -2,7 +2,11 @@
 #define GUARD_TEST_RUNNER_H
 
 extern const bool8 gTestRunnerEnabled;
+#if TESTING
 extern const bool8 gTestRunnerHeadless;
+#else
+#define gTestRunnerHeadless FALSE
+#endif
 extern const bool8 gTestRunnerSkipIsFail;
 
 #if TESTING

src/test_runner_stub.c

@@ -7,5 +7,7 @@ const bool8 gTestRunnerEnabled = FALSE;
 // The Makefile patches gTestRunnerHeadless as part of make test.
 // This allows us to open the ROM in an mgba with a UI and see the
 // animations and messages play, which helps when debugging a test.
+#if TESTING
 const bool8 gTestRunnerHeadless = FALSE;
+#endif
 const bool8 gTestRunnerSkipIsFail = FALSE;