GDScript: Fix error handler for tests

This changes the error message to be more clear on the output files and
also fixes an issue with the relative path of the offending file that
was not trimmed correctly.

modules/gdscript/tests/gdscript_test_runner.cpp

@@ -257,6 +257,7 @@ bool GDScriptTestRunner::make_tests() {
 
 	ERR_FAIL_COND_V_MSG(err != OK, false, "Could not open specified test directory.");
 
+	source_dir = dir->get_current_dir() + "/"; // Make it absolute path.
 	return make_tests_for_dir(dir->get_current_dir());
 }
 
@@ -361,11 +362,9 @@ void GDScriptTest::error_handler(void *p_this, const char *p_function, const cha
 			break;
 	}
 
-	builder.append("\n>> ");
-	builder.append(p_function);
-	builder.append("\n>> ");
+	builder.append("\n>> on function: ");
 	builder.append(p_function);
-	builder.append("\n>> ");
+	builder.append("()\n>> ");
 	builder.append(String(p_file).trim_prefix(self->base_dir));
 	builder.append("\n>> ");
 	builder.append(itos(p_line));