tests/test_http_media_cache: Add unit tests for res_http_media_cache

This patch adds unit tests for res_http_media cache, that covers nominal
creation and retrieval - and through them as well, staleness and deletion
checks. In addition, this patch adds tests that covers the interaction of
various HTTP headers, including Expires, Etag, and Cache-Control.

ASTERISK-25654

Change-Id: I2db101e307c863857fe416d6f5bf4cace9ac7cf5