Merge pull request #8838 from yang-g/test_credentials

build.yaml

test/cpp/end2end/async_end2end_test.cc

test/cpp/end2end/end2end_test.cc

test/cpp/interop/client.cc

test/cpp/interop/client_helper.cc

test/cpp/interop/interop_server.cc

test/cpp/interop/server_helper.cc

test/cpp/interop/stress_test.cc

test/cpp/util/create_test_channel.cc

test/cpp/util/create_test_channel.h

test/cpp/util/test_credentials_provider.cc

test/cpp/util/test_credentials_provider.h

tools/run_tests/sources_and_headers.json

...xproj/interop_server_helper/interop_server_helper.vcxproj