test: enable testing of grpc-js

[JustinBeckwith]

This starts enabling the tests with grpc-js. Only problem is.... they fail right now! @alexander-fenster are you cool with taking this PR over and getting the tests to pass?

[alexander-fenster]

Yes! From what I see, the only two tests that fail use unsupported API. As far as I know, the corresponding functionality now exists in @grpc/proto-loader, so @eoogbe or I will work on a code change to make it work (will be somewhat similar to #291, which was then reverted by #301).

[alexander-fenster]

With my last commit, unit tests should pass. To make system tests work, first of all we need to refactor grpc-js a little bit, it must not use getRequestMetadata from the auth library as it's deprecated, and we get deprecation warning.

https://github.com/grpc/grpc-node/blob/494b97ac1a58a942714c6fea0990ba690888f626/packages/grpc-js/src/index.ts#L55-L65

I will take care of that.

[alexander-fenster]

Related PR for grpc-js: grpc/grpc-node#723