test: enable testing for grpc-js

[kjin]

do not review

This change re-writes some tests (as well as the any_grpc.js helper file) to test the pure js client against the native server. It should fail until a few pending PRs (#128, #131, #132, and two other future ones) are landed.

Skipped tests

Tests for Google credentials and deadline propagation are currently skipped for the JS client.

large removals in credentials_test.js and surface_test.js

The large removal in credentials_test.js is code I had previously copied into credentials_api_test.js and forgotten to delete.

The large removal in surface_test.js is what is now surface_server_test.js.

any_grpc.js

I've changed this file to export requireAs* and runAs* (* means either client or server) functions. The former functions as require but any nested calls to require('grpc') will be intercepted to return the relevant implementation instead. The latter simply takes a function and runs it with a single argument, which is the relevant grpc implementation. This is primarily useful for separating client and server implementations in the same scope, as I can turn

// fake api for illustrative purposes

const server = grpc.createServer();
// do something with server

const client = grpc.createClient();
// do something with client

into

// runAs* allows us to re-define 'grpc' in the scope of the passed callback

anyGrpc.runAsServer((grpc) => {
  const server = grpc.createServer();
  // do something with server
});

anyGrpc.runAsClient((grpc) => {
  const client = grpc.createClient();
  // do something with client
});

This allows us to change tests back to using require('grpc') and be relatively unencumbered by changes specific to dual implementations of grpc.