llext: add a test for the pre_located feature

This test checks that the pre_located feature works as expected. It
creates a new extension that is manually relocated to a specific address
via the add_llext_command() CMake function invoking a custom linker
command. The test then loads the extension setting the pre_located
option and checks that the symbol is resolved properly.

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