Some test files fail if I run them seperately

[rishabh-arya95]

I was doing local development, If I execute some test files separately they fail with TypeError exception

example:

 app
   .engine(ext, fn)
     1) should map a template engine


 0 passing (12ms)
 1 failing

 1) app
      .engine(ext, fn)
        should map a template engine:
    Uncaught TypeError: Cannot read property 'equal' of undefined
     at /private/tmp/express/test/app.engine.js:25:20
     at /private/tmp/express/test/app.engine.js:10:5
     at FSReqCallback.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:73:3)

I see should has not been imported in some files so that is the reason, it would we better if we import it in the test files wherever required and not make files dependent on each other.