h2_http_proxy_test flaky for several tests under epoll1 and load

[dgquintas]

bins/msan/h2_http_proxy_test is flaky for compressed_payload and max_concurrent_streams (also for stream_compression_compressed_payload, but that's almost certainly due to the same cause as compressed_payload):

Example log for compressed_payload

...
I0905 11:48:58.247319712   63259 compressed_payload.c:46]    Running test: test_invoke_request_with_disabled_algorithm/chttp2/fullstack
I0905 11:48:58.251436408   63259 http_proxy_fixture.c:497]   Proxy address: localhost:6453
I0905 11:48:58.305924714   63800 http_connect_handshaker.c:304] Connecting to server localhost:17123 via HTTP proxy ipv6:[::1]:6453
D0905 11:48:58.306122605   63800 http_proxy_fixture.c:341]   on_read_request_done: 0x74400000d800 "No Error"
E0905 11:48:58.310335480   63800 call.c:1493]                Compression algorithm 'gzip' is disabled.
E0905 11:48:58.311058760   63259 cq_verifier.c:273]          cq returned unexpected event: OP_COMPLETE: tag:0x1 OK
E0905 11:48:58.311092455   63259 cq_verifier.c:280]          expected tags:
tag(101) GRPC_OP_COMPLETE success=1 test/core/end2end/tests/compressed_payload.c:194

Reproduction:
$ ./tools/run_tests/run_tests.py -l c -c msan -r 'h2_http_proxy_test' -n inf --force_use_pollers=epoll1 -t -a 20 -S

[dgquintas]

Happens also for hpack_size:

D0915 05:50:26.560001991     983 test_config.c:388]          test slowdown factor: sanitizer=5, fixture=1, poller=1, total=5
I0915 05:50:26.576386474     983 ev_epoll1_linux.c:93]       grpc epoll fd: 3
D0915 05:50:26.577441968     983 ev_posix.c:107]             Using polling engine: epoll1
D0915 05:50:26.578532991     983 dns_resolver.c:301]         Using native dns resolver
I0915 05:50:26.581387333     983 hpack_size.c:182]           Running test: test_size:e=4096:d=4096/chttp2/fullstack
I0915 05:50:26.613758817     983 http_proxy_fixture.c:497]   Proxy address: localhost:10286
I0915 05:50:26.656331234     983 http_connect_handshaker.c:305] Connecting to server localhost:13423 via HTTP proxy ipv6:[::1]:10286
D0915 05:50:26.656691281    1007 http_proxy_fixture.c:341]   on_read_request_done: 0x7d8c0000e000 "No Error"
E0915 05:50:51.869738963     983 cq_verifier.c:273]          cq returned unexpected event: OP_COMPLETE: tag:0x1 OK
E0915 05:50:51.869808402     983 cq_verifier.c:280]          expected tags:
tag(101) GRPC_OP_COMPLETE success=1 test/core/end2end/tests/hpack_size.c:303

[yashykt]

Will work on the hpack bug in #12787