add more integration test cases

[mileszhang2016]

Discussed in #968

^{Originally posted by kwanhur January 23, 2022}
when add some feature into bfe_server, it seems to add test case uncomfortably, needs to mock a mass of struct case for BfeServer, any other friendly ways?

[kwanhur]

There MockServer locate at bfe_util/mock_server, but seems lack relate usage doc.

I found two places had used it, bfe_http2/server_test and bfe_websocket.

When modification on bfe_server, good for a trial with the MockServer.

[mileszhang2016]

The requirement of "Add support for setting up context (e.g., bfe_config, bfe_server) to ease unit testing",
has been changed to "add more integration test cases"