net/http: TestServerKeepAlivesEnabled_h2 failure with "got 2 conns, want 1"

[bcmills]

--- FAIL: TestServerKeepAlivesEnabled_h2 (0.30s)
    serve_test.go:5768: request 1: got 2 conns, want 1
2021/12/03 17:42:33 http: TLS handshake error from 127.0.0.1:21794: read tcp 127.0.0.1:21793->127.0.0.1:21794: read: connection reset by peer
2021/12/03 17:42:33 http: TLS handshake error from 127.0.0.1:21796: read tcp 127.0.0.1:21793->127.0.0.1:21796: read: connection reset by peer
2021/12/03 17:42:33 http: TLS handshake error from 127.0.0.1:21797: read tcp 127.0.0.1:21793->127.0.0.1:21797: read: connection reset by peer
FAIL
FAIL	net/http	135.565s

greplogs --dashboard -md -l -e 'FAIL: TestServerKeepAlivesEnabled.*\n\s+.*got \d+ conns, want 1'

2021-12-03T14:28:11-a174638/freebsd-arm-paulzhol

This may be somehow related to #44420. (See previously #46707, #21724, #18701, #18127, #18083.)

(CC @neild)