Restore guards on a couple of tests

These tests check for failures, but can only fail where SIZE_MAX
exceeds some limit (UINT_MAX or UINT32_MAX) and do not fail
in this way otherwise - so guards are needed.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/suites/test_suite_pk.function b/tests/suites/test_suite_pk.function
index bafd87a..a99b6c0 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -1139,10 +1139,12 @@
     TEST_ASSERT(strcmp(mbedtls_pk_get_name(&alt), "RSA-alt") == 0);
 
     /* Test signature */
+#if SIZE_MAX > UINT_MAX
     TEST_ASSERT(mbedtls_pk_sign(&alt, MBEDTLS_MD_NONE, hash, SIZE_MAX,
                                 sig, sizeof(sig), &sig_len,
                                 mbedtls_test_rnd_std_rand, NULL)
                 == MBEDTLS_ERR_PK_BAD_INPUT_DATA);
+#endif /* SIZE_MAX > UINT_MAX */
     TEST_ASSERT(mbedtls_pk_sign(&alt, MBEDTLS_MD_NONE, hash, sizeof(hash),
                                 sig, sizeof(sig), &sig_len,
                                 mbedtls_test_rnd_std_rand, NULL)