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

include/gtest/internal/custom/gtest-port.h

@@ -39,6 +39,11 @@
 //     GTEST_DEFINE_int32_(name, default_val, doc)
 //     GTEST_DEFINE_string_(name, default_val, doc)
 //
+//   Test filtering:
+//     GTEST_TEST_FILTER_ENV_VAR_ - The name of an environment variable that
+//                                  will be used if --GTEST_FLAG(test_filter)
+//                                  is not provided.
+//
 //   Logging:
 //     GTEST_LOG_(severity)
 //     GTEST_CHECK_(condition)

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;
 }