Add some pings to force happens-before in SPDY tests.

Previously we were relying on accidents to get our tests to pass. There
may still be some that require work, but this should fix the three tests
that I saw fail.

https://github.com/square/okhttp/issues/525
Merge pull request #709 from square/jwilson_0417_some_spdy_flakiness

Add some pings to force happens-before in SPDY tests.