Move testing of mbedtls_blowfish_free() to separate test case

It should be tested regardless of the setting of MBEDTLS_CHECK_PARAMS.
diff --git a/tests/suites/test_suite_blowfish.function b/tests/suites/test_suite_blowfish.function
index 1d1422a..7a93cd1 100644
--- a/tests/suites/test_suite_blowfish.function
+++ b/tests/suites/test_suite_blowfish.function
@@ -7,6 +7,13 @@
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void blowfish_valid_param( )
+{
+    TEST_VALID_PARAM( mbedtls_blowfish_free( NULL ) );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_CHECK_PARAMS:!MBEDTLS_PARAM_FAILED_ALT */
 void blowfish_invalid_param( )
 {