Changes

start-testsuite: fix passing $TEST to ttcn3_start

Fix that testenv / ttcn3_start kept running after the testsuites were
already done. This was caused by passing an empty string to ttcn3_start
as test argument, which causes it to still use the config file, but run
in a single test mode:

https://gitlab.eclipse.org/eclipse/titan/titan.core/-/blob/9.0.0/mctr2/mctr/ttcn3_start#L307-319

After the first test ran, ttcn3_start sends "emtc" to the MTC, which
replies with "MTC cannot be terminated." as it is still in
MC_EXECUTING_TESTCASE instead of MC_READY:

https://gitlab.eclipse.org/eclipse/titan/titan.core/-/blob/9.0.0/mctr2/cli/Cli.cc#L695-700

The ttcn3_start script then waits forever for the "MTC terminated.."
string, which doesn't come since "emtc" isn't sent to the MTC a second
time:

https://gitlab.eclipse.org/eclipse/titan/titan.core/-/blob/9.0.0/mctr2/mctr/ttcn3_start#L353-356

Fixes: 050ba48c ("buildsystem: build out-of-tree")
Change-Id: Ic50de8350e20d101417c5689058b64fe6547126f