Use event polling instead of blocking when outputting to a file.

[glennw]

This stops some ref tests on linux blocking while waiting for
user input events and never outputting the file.

Also mark vertical writing test as flaky on all platforms. Due
to unrelated timing reasons, this fails reliably on mac when glutin
is enabled. The related bug with details on the root cause is #3926.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3568

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[glennw]

r? @larsbergstrom or @metajack

[larsbergstrom]

r+

[glennw]

r=larsbergstrom

[bors-servo]

saw approval from larsbergstrom
at glennw@8e32ba9

[bors-servo]

merging glennw/servo/glutin-tests = 8e32ba9 into auto

[bors-servo]

glennw/servo/glutin-tests = 8e32ba9 merged ok, testing candidate = b83b493

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/391
success: http://build.servo.org/builders/linux2/builds/390
success: http://build.servo.org/builders/android/builds/426
success: http://build.servo.org/builders/mac1/builds/607
success: http://build.servo.org/builders/mac2/builds/608

[bors-servo]

fast-forwarding master to auto = b83b493