Fix stalling of keytests when invalid DISPLAY

Before, the keytests would run infinitely if the
tests were run, e.g. through SSH without X forwarding.
To reproduce that problem, run

  DISPLAY='' ctest -R hello

This commit causes the test to correctly fail in such situations.
(Collaboration with Scott Kostyshak)
keytests: add a timeout for LyX process to start

If the LyX window does not appear after a certain (conservative)
time, the keytest now fails. One reason that caused the LyX window
to never show up was when DISPLAY was not set. This particular case
has been fixed in <commit1hash>. This commit is meant for the more general
scenario.
(Collaboration with Scott Kostyshak)
* sk.po