ztest: Handle assert failures without signal handler

The signal handler needs to return gracefully. When multiple
tests assert, the first assert will raise SIGABRT and the signal
handler will run and the test will stop running.
The second assert will raise SIGABRT but the signal handler
will not be called, therefore the test PASS/FAIL status is not
updated correctly.

Signed-off-by: Al Semjonovs <asemjonovs@google.com>