make test: add CACHE_OUTPUT option

This option allows one to toggle whether captured VPP stdout/stderr
output is printed inline (each line logged immediately after being
collected). Default setting is 0/no (behaviour same as before).

This allows one to see the output in case of vpp crashing during test
run even if test framework gets stuck waiting for shared memory mutex
(which crashed vpp can no longer unlock).

Complete stdout/stderr output is always printed as a block to the
logger after test finishes.

Change-Id: If3645a9b85562dba26ed9c87daa27ea5ef34f2e5
Signed-off-by: Klement Sekera <ksekera@cisco.com>