tests/subsys/llext: cleanup memory protection options

Most of the tests in the LLEXT subsystem test suite run with memory
protection hardware (MPU/MMU) disabled, to avoid including memory domain
issues while testing the core functionality. Only a few tests need to
run with MPU/MMU enabled.

This patch simplifies the testcase.yaml by setting the memory protection
as disabled in a shared config file that is included by most tests.

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