Code review comment for lp:~bjornt/launchpad/enable-windmill-again

Re-enable the Windmill tests in the default test suite. Remove the
filter for Windmill tests, and start a virtual framebuffer x server in
test_on_merge.py.

Last time we tried to enabled them we modified 'make check' to run
test_on_merge.py using xvfb-run. This didn't work, since the buildbot
slaves don't use 'make check'. The buildbot slaves should use 'make
check', but it's still better to put the xvfb-run call within
test_on_merge.py, in case there are other call sites.

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Björn Tillenius | https://launchpad.net/~bjornt