To help debug rare nondeterministic server_test failures, log Framer
reads and writes. Sample output from an injected test failure:

$ go test golang.org/x/net/http2
--- FAIL: TestServer_Request_Reject_Pseudo_scheme_invalid (0.00s)
    server_test.go:998: server request made it to handler; should've been rejected
    server_test.go:514: got a *http2.HeadersFrame; want *RSTStreamFrame
    server_test.go:229: Framer read log:
        2016-12-07 17:06:11.907199013 Framer 0xc4212665b0: read SETTINGS len=18, settings: MAX_FRAME_SIZE=1048576, MAX_CONCURRENT_STREAMS=250, MAX_HEADER_LIST_SIZE=1048896
        2016-12-07 17:06:11.907523124 Framer 0xc4212665b0: read SETTINGS flags=ACK len=0
        2016-12-07 17:06:11.908090453 Framer 0xc4212665b0: read HEADERS flags=END_STREAM|END_HEADERS stream=1 len=48
    server_test.go:235: Framer write log:
        2016-12-07 17:06:11.907152927 Framer 0xc4212665b0: wrote SETTINGS len=0
        2016-12-07 17:06:11.907207016 Framer 0xc4212665b0: wrote SETTINGS flags=ACK len=0
        2016-12-07 17:06:11.907550525 Framer 0xc4212665b0: wrote HEADERS flags=END_STREAM|END_HEADERS stream=1 len=16

Framer logs are written at the end of a test only if the test failed
and only if http2debug!=2. (If http2debug=2, then Framer logs are already
written to stdout.)

Hopefully this will help debug flaky tests. Or it might not.
The code is kind of ugly.

Updates golang/go#18235

Change-Id: I74c8ef82521d81f123663c98c883c71d9843964f
Reviewed-on: https://go-review.googlesource.com/34130
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>