Changes

FR testsuites: clean up all networks before start

Add a new network_clean_remove_all_ttcn3 function and use it in the fr
related testsuites to ensure no network is running before the test
starts. We just had the situation where the network link between
gtp0-deb10fr (where these testsuites run exclusively, and only one at a
time) and the jenkins host went down. And so the clean up trap
apparently did not run and starting a new test fails as the old network
still exists and has the network devices attached.

Related: OS#5802
Change-Id: I6a9464a11edcba978be08764bec9de19760a5c77