testsuite: ztest: Add logs flushing after each test case

When deferred mode is used, logging is using lowest priority
thread to process the logs. When test cases are performed one
by one processor never reaches lowest priority thread until
whole suite is completed. Added flushing after each test case.

Feature is optional (by default enabled).
CONFIG_TEST_LOGGING_FLUSH_AFTER_TEST=n to disable.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>