Also consolidate Tesla mocks into the HttpRequestMock module.
Tests were not exercising the real codepaths. The Rich Media Preview only works with https, but most of these tests were only mocking http.
Caching can't work in async tests, so for them it is mocked to a
null cache that is always empty. Synchronous tests are stubbed
with the real Cachex, which is emptied after every test.
Mark Felder