Correct definition of ENTROPY_HAVE_STRONG

Mbed TLS 2.1 doesn't have MBEDTLS_TEST_NULL_ENTROPY macro.
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index b80831e..5c20f81 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -59,11 +59,10 @@
 
 /* Indicates whether we expect mbedtls_entropy_init
  * to initialize some strong entropy source. */
-#if defined(MBEDTLS_TEST_NULL_ENTROPY) ||             \
-    ( !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES) && \
-      ( !defined(MBEDTLS_NO_PLATFORM_ENTROPY)  ||     \
-         defined(MBEDTLS_HAVEGE_C)             ||     \
-         defined(MBEDTLS_ENTROPY_HARDWARE_ALT) ) )
+#if !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES) &&     \
+    ( !defined(MBEDTLS_NO_PLATFORM_ENTROPY)  ||         \
+       defined(MBEDTLS_HAVEGE_C)             ||         \
+       defined(MBEDTLS_ENTROPY_HARDWARE_ALT) )
 #define ENTROPY_HAVE_STRONG
 #endif