tests: adds test to verify context during concurrent request

[kurtharriger]

Adds a test for concurrent request that ensures that content is correctly preserved over promises when using cls-hooked.

If you change

ctxImpl = new CLSContext('zipkin', true); 

to

ctxImpl = new CLSContext('zipkin', false);

the test will fail.

[jcchavezs]

Thanks a lot @kurtharriger, could you please also add the failing case?

[jcchavezs]

I'll add the test case myself. Thanks a lot @kurtharriger!