net/http: TestServerRequestContextCancel_ConnClose timeouts on slower builders

[bcmills]

--- FAIL: TestServerRequestContextCancel_ConnClose (4.92s)
    serve_test.go:4897: timeout waiting to see ServeHTTP get called
2022/04/13 07:12:39 http: TLS handshake error from 127.0.0.1:51046: EOF
2022/04/13 07:12:39 http: TLS handshake error from 127.0.0.1:51048: EOF
2022/04/13 07:12:39 http: TLS handshake error from 127.0.0.1:51052: EOF
2022/04/13 07:12:39 http: TLS handshake error from 127.0.0.1:51058: EOF
FAIL
FAIL	net/http	66.927s

greplogs --dashboard -md -l -e 'FAIL: TestServerRequestContextCancel_ConnClose .*\n .*timeout waiting to see ServeHTTP get called'

2022-04-12T23:50:58-65558a4/linux-riscv64-jsing
2021-03-24T21:51:22-29ed12d/darwin-amd64-race
2020-10-01T19:13:03-5756b35/darwin-amd64-race
2019-11-08T23:22:06-11da2b2/solaris-amd64-oraclerel

[bcmills]

Looks like another hard-coded arbitrary timeout in the test. 😩

The problem does not appear to be platform-dependent, and the fix should be easy, so marking as release-blocker for Go 1.19.

[gopherbot]

Change https://go.dev/cl/400816 mentions this issue: net/http: eliminate arbitrary timeouts in TestServerRequestContextCancel_ConnClose