Wait for dummy X server to shut down in Environment::TearDown()
Wait for dummy X server to shut down in Environment::TearDown()

If two xorg-gtest binaries are executed one after another, the second
one may attempt to start its X server before the first one's X server
has fully shut down. This leads to the second X server not starting.

Signed-off-by: Chase Douglas <69e02ec421aa70a149511825d897f2549ceee9a2@canonical.com>
Reviewed-by: Stephen M. Webb <8cce60aa4861c475af3c10231a6f3a1b25943bd6@bregmasoft.ca>

freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest