ztest: Add zassume* API

Add an assume API which works like JUnit's. Assumptions can be made
at any point before your test returns (setup, before, and during the
test). If an assumption fails, the test will be marked as skipped.

This helps avoid a cascading affect of failed tests when a base
feature breaks. The feature is expected to have tests and the tests
which depend on it should be skipped (not failed) if that feature
is broken.

Issue #42472

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