tests: run tests against a running VPP

tests: run tests against a running VPP

Usage:
test/run.py -r  -t {test_filter}
Instead of starting a new instance of VPP, when the -r argument
is provided, test is run against a running VPP instance. Optionally,
one can also set the VPP socket directory using the -d
argument. The default location for socket files is
/var/run/user/${uid}/vpp and /var/run/vpp if VPP is started
as root.

Type: improvement

Change-Id: I05e57a067fcb90fb49973f8159fc17925b741f1a
Signed-off-by: Naveen Joy <[email protected]>