net/http: TestOptions/h1 failures

```
#!watchflakes
default <- pkg == "net/http" && test == "TestOptions/h1"
```

Issue created automatically to collect these failures.

Example ([log](https://ci.chromium.org/b/8738971195910692529)):

=== RUN TestOptions/h1
=== PAUSE TestOptions/h1
=== CONT TestOptions/h1
serve_test.go:3682: Get "http://127.0.0.1:53667/second": net/http: HTTP/1.x transport connection broken: http: CloseIdleConnections called
--- FAIL: TestOptions/h1 (0.03s)

— [watchflakes](https://go.dev/wiki/Watchflakes)

Start with the net/http TestOptions/h1 test and the failure at serve_test.go:3682. Reproduce the watchflakes failure, then trace the interaction with CloseIdleConnections; done means the intermittent HTTP/1.x transport failure no longer occurs and the test passes reliably.

Written by the indexing model from the issue text.