Eliminate GTEST_TEST_FILTER_ENV_VAR_.

GTEST_TEST_FILTER_ENV_VAR_ was used to specify an environment variable to obtain
the default test filter from. By default it was unset which broke
"--test_filter" for bazel. This CL eliminates GTEST_TEST_FILTER_ENV_VAR_ and
explicitly obtains the default test filter from the environment variable
TESTBRIDGE_TEST_ONLY if it exists.

googletest/test/gtest_testbridge_test_.cc

@@ -0,0 +1,44 @@
+// Copyright 2018, Google LLC.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: rfj@google.com (Rohan Joyce)
+
+// This program is meant to be run by gtest_test_filter_test.py.  Do not run
+// it directly.
+
+#include "gtest/gtest.h"
+
+// These tests are used to detect if filtering is working. Only
+// 'TestThatSucceeds' should ever run.
+
+TEST(TestFilterTest, TestThatSucceeds) {}
+
+TEST(TestFilterTest, TestThatFails) {
+  ASSERT_TRUE(false) << "This test should never be run.";
+}