lint: replace require with import in test #659

remove /* eslint-disable @typescript-eslint/no-var-requires */ in test.

It needs more changes than just making _innerApiCalls public. LROperation, gax.CallOptions ... are incompatible with the real types of method calls. Some of them can be forced compatible but lots of as unknown as someType will be used. As @alexander-fenster suggested, we should move unit test to sinon for removing custom mock codes.

ref: googleapis/gapic-generator-typescript#343