llext: tests: add zassert macros everywhere

Most of the current llext tests did not actually check the results of
the tests, so the CI reported a pass even if something was wrong when
looking at the logs. This patch adds the appropriate zassert_* macros to
all the tests, to ensure that they actually perform correctly.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>