Add new global fixture to mock HTTP requests

[ofek]

What does this PR do?

This adds a new global fixture that returns a function used to mock HTTP requests for the lifetime of a test

Motivation

I am using this in a feature branch and realized we often need this

Example

def test(mock_http_response):
    mock_http_response(
        """
        # HELP go_memstats_alloc_bytes Number of bytes allocated and still in use.
        # TYPE go_memstats_alloc_bytes gauge
        go_memstats_alloc_bytes 6.396288e+06
        """
    )
    ...