make test: add scripts for easy test looping

Allows easy running of test(s) in a loop with configurable
action (e.g. git pull) run between test runs and possible
email notification on failure.

Usage:

test-loop.sh [-p <pre-exec-cmd>] [-m <email>] -- <make test options>

Example:

Run 'make test-debug' in a loop until a failure is encountered,
upon which an email is fired to ksekera@cisco.com. In between test
runs, update the workspace using via 'git pull' and if anything changed,
perform 'git clean' before running another 'make test-debug':

test/scripts/test-loop.sh -p test/scripts/git_pull_or_clean.sh \
-m ksekera@cisco.com -- test-debug

Change-Id: I114321c6c152d2c7e181e915fc8c51aab1ff3693
Signed-off-by: Klement Sekera <ksekera@cisco.com>