Rename the 'no entropy' feature to MBEDTLS_TEST_NULL_ENTROPY

Following review and for clarity, changed the name of the feature to 'null
entropy'.
diff --git a/library/version_features.c b/library/version_features.c
index a00ae27..37b30a9 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -159,9 +159,9 @@
 #if defined(MBEDTLS_AES_DECRYPT_ALT)
     "MBEDTLS_AES_DECRYPT_ALT",
 #endif /* MBEDTLS_AES_DECRYPT_ALT */
-#if defined(MBEDTLS_TEST_WO_ENTROPY)
-    "MBEDTLS_TEST_WO_ENTROPY",
-#endif /* MBEDTLS_TEST_WO_ENTROPY */
+#if defined(MBEDTLS_TEST_NULL_ENTROPY)
+    "MBEDTLS_TEST_NULL_ENTROPY",
+#endif /* MBEDTLS_TEST_NULL_ENTROPY */
 #if defined(MBEDTLS_ENTROPY_NV_SEED)
     "MBEDTLS_ENTROPY_NV_SEED",
 #endif /* MBEDTLS_ENTROPY_NV_SEED */