refactor(typescript): noImplicitAny for subscription test file

[praveenqlogic]

towards #313 (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[JustinBeckwith]

Still seeing some weird stuff here with grpc-js @callmehiphop @alexander-fenster

 1) pubsub
       Subscription
         should ack the message:
     Error: stream.push() after EOF (and Mocha's done() called multiple times)
      at readableAddChunk (_stream_readable.js:240:30)
      at ClientDuplexStreamImpl.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.call.on (node_modules/@grpc/grpc-js/build/src/call.js:36:21)
      at addChunk (_stream_readable.js:263:12)
      at readableAddChunk (_stream_readable.js:250:11)
      at Http2CallStream.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.handleFilteredRead (node_modules/@grpc/grpc-js/build/src/call-stream.js:75:23)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:189:7)
  2) pubsub
       Subscription
         should ack the message:
     Error: stream.push() after EOF (and Mocha's done() called multiple times)
      at readableAddChunk (_stream_readable.js:240:30)
      at ClientDuplexStreamImpl.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.call.on (node_modules/@grpc/grpc-js/build/src/call.js:36:21)
      at addChunk (_stream_readable.js:263:12)
      at readableAddChunk (_stream_readable.js:250:11)
      at Http2CallStream.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.handleFilteredRead (node_modules/@grpc/grpc-js/build/src/call-stream.js:75:23)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:189:7)
  3) pubsub
       Subscription
         should nack the message:
     Error: stream.push() after EOF (and Mocha's done() called multiple times)
      at readableAddChunk (_stream_readable.js:240:30)
      at ClientDuplexStreamImpl.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.call.on (node_modules/@grpc/grpc-js/build/src/call.js:36:21)
      at addChunk (_stream_readable.js:263:12)
      at readableAddChunk (_stream_readable.js:250:11)
      at Http2CallStream.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.handleFilteredRead (node_modules/@grpc/grpc-js/build/src/call-stream.js:75:23)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:189:7)
  4) pubsub
       Subscription
         should nack the message:
     Error: stream.push() after EOF (and Mocha's done() called multiple times)
      at readableAddChunk (_stream_readable.js:240:30)
      at ClientDuplexStreamImpl.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.call.on (node_modules/@grpc/grpc-js/build/src/call.js:36:21)
      at addChunk (_stream_readable.js:263:12)
      at readableAddChunk (_stream_readable.js:250:11)
      at Http2CallStream.Readable.push (_stream_readable.js:208:10)
      at Http2CallStream.handleFilteredRead (node_modules/@grpc/grpc-js/build/src/call-stream.js:75:23)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:189:7)
  5) pubsub
       Subscription
         should respect flow control limits:
     Error: done() called multiple times
      at _subscriber.close.then (src/subscription.ts:304:41)
      at <anonymous>
  6) pubsub
       Subscription
         should respect flow control limits:
     Error: done() called multiple times
      at _subscriber.close.then (src/subscription.ts:304:41)
      at <anonymous>

[callmehiphop]

The Error: stream.push() after EOF errors look like a grpc error to me.