tests: bluetooth: tester: Increase number of PSA key slots for mesh

MESH PTS tests require different number of keys, as an example; only
one netkey requires 8 key slots and there are test cases that use at
least two netkeys which consume the default 16 slots, so it is better
to have enough slots all times. Setting slot count to 32.

Signed-off-by: alperen sener <alperen.sener@nordicsemi.no>