googletest

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.