x/net/http2: test timeouts on js-wasm builder

[bcmills]

2019-12-09T16:08:50-c0dbc17/js-wasm has an interesting timeout failure.

It's not obvious to me whether it is due to a bug in the test, a bug in the js/wasm standard library, or a bug in x/net/http2 or one of its dependencies that happens to be exposed by the different goroutine scheduling on js/wasm.

The presence of a goroutine blocked in net.(*bufferedPipe).Read via crypto/tls makes me suspect a concurrency bug in either golang.org/x/net/http2 or crypto/tls, but admittedly that evidence is not compelling.

I also can't reliably determine how often this occurs or on what platforms because I can't fetch most of the x/net logs (#35515).

CC @bradfitz @FiloSottile @tombergan

[bcmills]

The earliest of these failures I could find by manual inspection of the dashboard is https://build.golang.org/log/8d467bcc05d2e38b1b903835bb15858dfc547a62, at CL 208798.

[bcmills]

That's also in TestIssue20704Race rather than TestServer_Push_RejectAfterGoAway, so retitling.

The failure mode is otherwise the same: stuff blocked in golang.org/x/net/http2.readFrameHeader and crypto/tls.(*Conn).Read, and runnable goroutines otherwise. Maybe the tests are just too slow overall on that builder.