plugins/python: memleak fixes in test

The main problem was that string array objects were constructed
differently:
- if constructed by the test, then the elements were constant
- if constructed by the plugin, then the elements were allocated

Modified it so that now each array contains allocated strings so
they can be handled similarly. For freeing, I have used the
str_array_free function from the plugin, so I have linked its object
into the test runner.

Happy path is now free of "definitely lost" memleaks, so the test
can be used for valgrind.

plugins/python/regress/testhelpers.h

@@ -166,7 +166,7 @@ int verify_str_set(char **actual_set, char **expected_set, const char *actual_va
     do { \
         char **expected_set = create_str_array(__VA_ARGS__); \
         VERIFY_TRUE(verify_str_set(actual_set, expected_set, #actual_set)); \
-        free(expected_set); \
+        str_array_free(&expected_set); \
     } while(false)
 
 #endif // PYTHON_TESTHELPERS