tfm: Update TF-M regression tests sample to build NS app

Update the TF-M regression tests sample to build the NS app
in the tf-m-tests repository as an external project.

The regression tests need to provide test configurations to both
TF-M an NS app.
Duplicate configuration done in the spe/CMakeLists.txt to configure
TF-M image for the regression tests.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
Signed-off-by: Markus Swarowsky <markus.swarowsky@nordicsemi.no>