ztest: Allow 'before' functions to run in privilaged mode

When writing a test suite, it's more common to want the 'before'
hook to run in privilaged mode, even when the test is run in userspace.
Reconfigure ztest to first run the test thread callback in privilaged
mode and only enter userspace after the test rule and suite's 'before'
functions ran.

Signed-off-by: Yuval Peress <peress@google.com>