This commit decouples the http mock conversation
feature from assert_bot_response(), and moves it
to the context manager mock_http_conversation().
This allows a modular design with context managers
that could be added for assert_bot_response().
This commit integrates the mock_test function in
check_expected_responses and makes this function usable
for simple tests only by not allowing mock http conversations.
assert_bot_response() is now used for single message-response pairs,
resolving potential issues with multiple messages and a single http
request-response pair.
The giphy bot test file is updated accordingly.
