Rename internal MBEDTLS_ENTROPY_HAVE_STRONG to ENTROPY_HAVE_STRONG

This commit renames the test-only flag MBEDTLS_ENTROPY_HAVE_STRONG to ENTROPY_HAVE_STRONG to make it more transparent
that it's an internal flag, and also to content the testscript tests/scripts/check-names.pl which previously complained
about the macro occurring in a comment in `entropy.c` without being defined in a library file.
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index 39cd3c7..d367467 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -113,7 +113,7 @@
          defined(MBEDTLS_HAVEGE_C)             ||     \
          defined(MBEDTLS_ENTROPY_HARDWARE_ALT) ||     \
          defined(ENTROPY_NV_SEED) ) )
-#define MBEDTLS_ENTROPY_HAVE_STRONG
+#define ENTROPY_HAVE_STRONG
 #endif