feat(ngMock): add flushByIndex function

Request Type: feature

How to reproduce: 

Component(s): ngMock

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

I've recently needed to flush httpBackend requests in an arbitrary order to add unit tests around an edge case. This PR satisfies that need.

**Other Comments:**

Adds a flushByIndex function to allow pending requests on httpBackend to be flushed in an arbitrary
order.