Unit tests for jQuery.grep (#14325)

Added 7 unit tests for jQuery.grep(). These include tests for grep() on:
* empty array
* sparse array
* an array that contains elements of interest 
   - with inversion flag not passed
   - inversion flag passed in as false
   - inversion flag true

* an array that has _no_ elements of interest
   - with inversion flag not passed
   - inversion flag passed in as false
   - inversion flag true