tool/run: can't cancel 'run_genode_until forever'

[Martin Stein]

I'm not sure if this should be a github issue:

When executing a run test that does 'run_genode_until forever'
and the test stucks irrecoverable somewhere it's not possible
for me to cancel the process via [strg + c] in contrast to the
same situation with 'run_genode_until "STRING" TIMEOUT'.

I've tested this with terminator 0.96 and xfce4-terminal 0.6.1.
It's not a big issue but doing 'ps' and 'kill' all the time is
annoying and i'm not so skilled with expect to fix it quickly.

Martin