ztest: run before function in test thread

A lot of tests need to be able to get their current tid and do some
action with it. It makes sense for the `before` function/rule to be
able to run in the same thread as the test. Note that the `after`
function does not run in the same thread because we need to guarantee
that it will run.

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