Tests: tests for "zero size buf" alerts in unbuffered proxying.

Starting with nginx 1.19.1, non-buffered simple proxying of responses
with expected length 0 but some extra data might result in zero size
buffers being generated and "zero size buf" alerts in writer (ticket #2117).
Tests for a similar bug in FastCGI proxying are in 1586:e145509cc6eb.