tests: ztests: error_hook: Enable assert test mode

This commit enables the assert test mode (`CONFIG_ASSERT_TEST`) for the
ztest error hook test because it implements a custom post assert fail
hook (`ztest_post_assert_fail_hook`) that returns without aborting to
faciliate the testing of the assert functions.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>