Changes

testenv: add a --fast argument

The ttcn3-{tcpdump,dumpcap}-*.sh scripts can easily spend 5s or more for
each test with:
* Waiting until the pcap gets created.
* Waiting until pcap size does not increase anymore after the test.
* Saving the talloc report.

Add --fast to skip all this, to allow running a whole testsuite
significantly faster locally to check for regressions. Developers can
then still run failing tests again without --fast to get the pcap and
talloc reports.

Change-Id: Ief3488293e5c5c1a1c98244f80ffd56eadb040c5