net/http: TestServerShutdown_h1 failure on solaris-amd64-oraclerel builder

[2020-04-07T19:55:05-bfd569f/solaris-amd64-oraclerel](https://build.golang.org/log/5a09341d9fbc8460e8d8e6c9362d5be7fba5a17d)

```
--- FAIL: TestServerShutdown_h1 (0.11s)
    serve_test.go:5593: second request should fail. server should be shut down
FAIL
FAIL	net/http	16.646s
```

Not obvious to me whether this is a test timing dependency, kernel bug, or bug in net/http.

CC @bradfitz 