test_suite_psa_crypto: use finer grained checks on the key slot buffer size

Instead of skipping some tests when !MBEDTLS_PSA_STATIC_KEY_SLOTS,
add a proper check in the depends_on to verify if
MBEDTLS_PSA_KEY_BUFFER_MAX_SIZE is actually large enough to contain
the key used in such test.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

include/psa/crypto_extra.h

@@ -42,6 +42,13 @@ extern "C" {
      PSA_EXPORT_KEY_PAIR_OR_PUBLIC_MAX_SIZE : PSA_CIPHER_MAX_KEY_LENGTH)
 #endif /* !MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE*/
 
+/* Define the size of the each key slot buffer. */
+#if defined(MBEDTLS_PSA_STATIC_KEY_SLOTS)
+#define MBEDTLS_PSA_KEY_BUFFER_MAX_SIZE     MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE
+#else
+#define MBEDTLS_PSA_KEY_BUFFER_MAX_SIZE     SIZE_MAX
+#endif
+
 /** \addtogroup attributes
  * @{
  */