llext: add a test for llext_find_section

Add a new test case that verifies that llext_find_section() returns the
correct offset for a symbol in a loadable extension. This exploits the
fact that in the STORAGE_WRITABLE cases, the symbol addresses will be
inside the ELF file buffer, so they can be easily compared.

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