Changes

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

This reverts commit 57f680439f378f5bb5c6582297ae0636db6650f8.

We're observing a lot of turbulence in ttcn3-bts-test due to the
clock instability:

  Shutting down BTS, exit 1, reason: PC clock skew too high

Even though it's no longer necessary to respawn osmo-bts-trx,
let's restore the respawn.sh to recover from such failures.

Related: OS#6794
Change-Id: I1177e9068c49bf31a4010cd21700839dfa8d5050