tests: secure_storage: psa: enable malloc arena for Mbed TLS

The PSA Crypto implementation provided by Mbed TLS generally requires
some amount of heap to function, so enable the arena unconditionally
to make the test compatible with MINIMAL_LIBC configurations where the
arena is disabled by default.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>