sample: tfm_regression_test: Set library model explicitly

Set the TF-M library mode explicitly instead disabling IPC model and
relying on this selecting Libray model in the choice.
This is a follow-up on the TFM_IPC being put into a choice selection
when SFN model was added.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>