Shows object diff when an async assertion fails

[codefromthecrypt]

Before, the assertion failure that mocha could readily interpret was
wrapped when inside an async hook. This made for very not helpful
failures.

This unwraps to prevent this.

before:

(node:70680) UnhandledPromiseRejectionWarning: AssertionError: expected { Object (http.path, http.status_code) } to deeply equal { Object (http.path, http.status_code) }

after:

  1) axios instrumentation - integration test
       should support request shorthand (defaults to GET):

      Uncaught AssertionError: expected { Object (http.path, http.status_code) } to deeply equal { Object (http.path, http.status_code) }
      + expected - actual

       {
         "http.path": "/weather/wuhan"
      -  "http.status_code": "202"
      +  "http.status_code": "2022"
       }

See mochajs/mocha#2797