Changes

ttcn3-bts-test: do not use respawn.sh

Re-spawning osmo-bts is no longer necessary.  For quite some time now,
it has been able to attempt OML link re-establishment in a loop instead
of exit()ing.

Additionally, using respawn.sh makes crashes harder to detect.  Whether
we should restart the IUT process and continue executing the remaining
test cases after a crash is debatable.  However, none of the other
testsuites (except ttcn3-pcu-test, which actually requires restarting
osmo-pcu after each TC) use respawn.sh for that purpose.

Change-Id: I770a0530f90c5e03d2073bfc1c4cfd0e840379c9
Related: OS#6728, OS#6729