t Add unit testing to Godot using DocTest and added to GitHub Actions CI

Implements exit codes into the engine so tests can return their statuses.
Ideally we don't do this, and we use FIXUP logic to 'begin' and 'end' the engine execution for tests specifically.

Since realistically we're initialising the engine here we don't want to do that, since String should not require an engine startup to test a single header.

This lowers the complexity of running the unit tests and even for
physics should be possible to implement such a fix.

platform/linuxbsd/godot_linuxbsd.cpp

@@ -41,6 +41,9 @@ int main(int argc, char *argv[]) {
 
 	setlocale(LC_CTYPE, "");
 
+	// We must override main when testing is enabled
+	TEST_MAIN_OVERRIDE
+
 	char *cwd = (char *)malloc(PATH_MAX);
 	ERR_FAIL_COND_V(!cwd, ERR_OUT_OF_MEMORY);
 	char *ret = getcwd(cwd, PATH_MAX);