net/http: deflake new TestInterruptWithPanic_h2

net/http: deflake new TestInterruptWithPanic_h2

TestInterruptWithPanic_h2 was added yesterday in
https://golang.org/cl/33099 and https://golang.org/cl/33103

Deflake it. The http2 server sends an error before logging.

Rather than reorder the http2 code to log before writing the RSTStream
frame, just loop for a bit waiting for the condition we're
expecting.

This goes from 2 in 500 flakes for me to unreproducible.

Change-Id: I062866a5977f50c820965aaf83882ddd7bf98f91
Reviewed-on: https://go-review.googlesource.com/33140
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>