HTTP/2: Add more tests for flow control

[EvgeniiMekhanik]

We should check that tempesta:

• [x] Doesn't block connection and continue to forward Headers frames while flow control limit is exhausted.
• [x] Doesn't truncate and suspend any frames except DATA ones on reaching close to flow control limit.
• [x] ~curl with CURLPIPE_MULTIPLEX should correctly fetch many resources~ t_stress.test_stress.H2CurlStress

This cases should work after https://github.com/tempesta-tech/tempesta/issues/1196

• [x] Doesn't send more than allowed by per-connection flow control