test: tests for gRPC streaming proxy

[bisakhmondal]

What this PR does / why we need it:

In continuation with api7/grpc_server_example#12 this PR adds CLI tests for three different gRPC streaming method.

Closes #3601

Note: CLI tests will be broken until the linked PR is merged and either we release a new api7/grpc_server_example release or create a git submodule pointing the same upstream.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[bisakhmondal]

Abandoning this PR.

We'll test it through the usual Test Nginx approach, not with a shell script.