samples: tfm: tfm_psa_test: Add temporary isolation workaround

Add a workaround which sets the isolation level to 2 in CMake.
The TF-M tests require the large TF-M profile because it supports
the full list of crypto algorithms needed, not because of the
solation level.

For the TF-M tests the isolation level is irrelevant so we
set it to 2 here so that we don't exclude the platforms which
don't support the isolation level 3.

I communicated with a TF-M maintainer and he informed me that this
workaround will be included in the TF-M 2.2 release branch later
so this is a short lived workaround that can be reverted when the
relevant commit from the TF-M branch is cherry-picked.

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>