Mark Spdy3ConnectionTest#serverSendsSettingsToClient as known failure.

This is the second attempt to submit this CL. The first attempt had a
trailing comma on line 1409, which broke ART build bots.

This test is flaky both on Android Pie and the latest development branch.
I haven't looked too deeply into _why_ it is failing, so it _could_
indicate a serious problem, but since the test on the surface is testing
functionality that isn't used on Android, it might not be worth
investigating. This CL marks the test as a known failure.

Bug: 116289659
Bug: 138134183
Test: art/tools/buildbot-build.sh --host -j100 ; \
      art/tools/run-libcore-tests.sh --mode=host --variant=X32 --debug

Change-Id: Ic0b2f3d9ef8a76bb6e8f55c401cf87a8a806572a
(cherry picked from commit 6023ab703e7a6b4dd685d0ae06af846a3ab66941)