Set DISPLAY variable for keytests

I don't set it globally because I want something to go wrong if
it's because of the DISPLAY variable. It should only be needed
for the keytests.

keyword: autotests
Set a larger key delay for keytest.py

The autotests are now run with XVKBD_DELAY=150.
This fixes the occasional failing of keytest/findadv-20 for
me and for Kornel.

I'm surprised that this only recently became an issue. I wonder if
LyX's advanced find got a microsecond slower, either because of the
wrap-around implementation or because of changes to the interrupt
(when advanced find is searching it looks for the escape key event).
I will not investigate this theory because I could not reproduce the
problem manually so even if this is the case the user should be
unaffected.