Add tests to mock_tracer

[jhorowitz]

Mock tracer has no tests which is important for something which is used to test other components.

[jhorowitz]

I'm happy to take this one.

[yurishkuro]

at minimum we should have this in unittest.js

import { MockTracer } from '../mock_tracer';

// Run the tests on the Mock Tracer.
describe('MockTracer Compatibility', () => {
    apiCompatibilityChecks(() => new MockTracer());
});

However, this currently fails because mock tracer does not implement propagation, which it does in other languages (otherwise it's not possible to use MockTracer for testing framework instrumentations).