tests/llext: refactor: rename perm_setup to test_setup, add test_cleanup

This commit renames the perm_setup callback to test_setup and provides
the extension as an additional parameter. It also adds a test_cleanup
callback that is called after each test completes.

Setup and cleanup functions are now called regardless of whether
CONFIG_USERSPACE is enabled or not.

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