Change GetDefaultFilter to allow for the injection of a custom filter.

src/gtest.cc

@@ -189,6 +189,12 @@ bool g_help_flag = false;
 }  // namespace internal
 
 static const char* GetDefaultFilter() {
+#ifdef GTEST_TEST_FILTER_ENV_VAR_
+  const char* const testbridge_test_only = getenv(GTEST_TEST_FILTER_ENV_VAR_);
+  if (testbridge_test_only != NULL) {
+    return testbridge_test_only;
+  }
+#endif  // GTEST_TEST_FILTER_ENV_VAR_
   return kUniversalFilter;
 }