testsuite: support reboot to retry intermittent tests

When a test fails intermittently there is currently no alternative to
looking at logs and pressing a hardware reset button.  This commit
adds a Kconfig option that can be set when diagnosing an intermittent
failure.  The behavior is to do a cold reset of the board when the
test passes.  A counter is maintained in noinit memory to track the
number of times it takes to reproduce a failure.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>