llext: rework test and sample coverage

This patch reworks the testcase.yaml and sample.yaml files for the llext
subsystem to further reduce the number of tests performed by CI while
improving overall coverage.

The following changes are introduced by this commit:

- Remove the arch_allow field from the common section to allow any arch
  to be tested in the build_only test. All other tests explicitly narrow
  down the arches they are applicable to.
- In addition to platforms with active issues, also exclude a number of
  platforms that are always skipped by the runtime filter due to
  RAM/Flash limitations.
- Add integration_platforms to limit the test count to a few selected
  platforms which are representative of the different arches.
- Remove a number of duplicate SLID tests and group them into a single
  test that covers both ARM and Xtensa architectures.
- Test the relocatable case on ARM as well.

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