llext: set integration platform on relocatable test

This particular test was ignored in CI because, when using
platform_key, Twister picks the specific platform very early in
the process and it is possible that the chosen one is later
filtered out. To fix this, use the `integration_platforms` key
instead to specify the platforms where the test is expected to
complete successfully.

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