Add testHttpEngine_EnableHttpCache

New test for enable caching API. CtsTestServer doesn't add a
`cache-control: max-age=X` header, this stops Cronet HttpEngine from
caching the response. Use a non-hermetic server to circumvent that.

Test: m & test run
Change-Id: I6852c72590146892a3175ce8c23668e3abda0dbd