make test: detect hung tests

make test: detect hung tests

Run tests in a forked process with a set of pipes to communicate
keep-alives and overall result. This allows us to detect when e.g.
vpp dies mid-API call causing the test to hang waiting for response
(which will never come since vpp died).

Support setting a (per test case) TIMEOUT make test option to set timeout,
with a default timeout of 120 seconds.

Example - fail the test suite if any test-case fails to finish within
300s:

make TIMEOUT=300 test

Change-Id: I0d04f26a7232968f4bf043adf5d5b508f5018717
Signed-off-by: Klement Sekera <ksekera@cisco.com>