samples|tests: secure-storage: allow all test rng sources

Add MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG to the list of
test Kconfigs. Typical use case of this:

- the platform has "zephyr,entropy" defined in the devicetree which means
  that CSPRNG_AVAILABLE is set;
- for some reason the board actually does not have any entropy driver
  enabled (ex: ENTROPY_BT_HCI is disabled in nrf5340bsim when BT is _not_
  enabled in the build);
- since TEST_RANDOM_GENERATOR is enabled then also TEST_CSPRNG_GENERATOR
  would be;
- unfortunately the call to psa_generate_random() would fail in
  get_random_data() (in modules/mbedtls/zephyr_entropy.c) because
  MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG is not set and it would
  not reach "subsys/random/random_test_csprng.c".

This commit fixes this problem.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>