make test: improve stability

Disable automatic garbage collection and run it manually before
running each test case to minimize stalls. Improve vpp subprocess
cleanup. Reduce helper thread count to one and properly clean that
thread once it's not needed.

Change-Id: I3ea78ed9628552b5ef3ff29cc7bcf2d3fc42f2c3
Signed-off-by: Klement Sekera <ksekera@cisco.com>

test/util.py

@@ -1,3 +1,5 @@
+""" test framework utilities """
+
 import socket
 import sys
 from abc import abstractmethod, ABCMeta