Add some tests to increase coverage, fix some failures

src/node/ext/call.cc

src/node/interop/interop_client.js

src/node/src/credentials.js

src/node/test/call_test.js

src/node/test/channel_test.js

src/node/test/credentials_test.js

src/node/test/health_test.js

src/node/test/interop_sanity_test.js

src/node/test/surface_test.js

tools/run_tests/run_node.sh