TinyCrypt Test: Add guards to ECDSA verify tests from PK test suite

- The underlying test vectors are for Secp192r1, while TinyCrypt uses Secp256r1.
- The test implementation is specific to the structure of legacy-ECC PK ctxs.

Addition of analogous tests for TinyCrypt-based ECC PK contexts are left
for a later commit.

tests/suites/test_suite_pk.function

@@ -669,7 +669,7 @@ exit:
 }
 /* END_CASE */
 
-/* BEGIN_CASE depends_on:MBEDTLS_ECDSA_C */
+/* BEGIN_CASE depends_on:MBEDTLS_ECDSA_C:!MBEDTLS_USE_TINYCRYPT */
 void pk_ec_test_vec( int type, int id, data_t * key, data_t * hash,
                      data_t * sig, int ret )
 {