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.c

@@ -64,7 +64,8 @@ create_str_array(size_t count, ...)
 
     char ** result = calloc(count, sizeof(char *));
     for (size_t i = 0; i < count; ++i) {
-        result[i] = va_arg(args, char *);
+        const char *str = va_arg(args, char *);
+        result[i] = (str == NULL ? NULL : strdup(str));
     }
 
     va_end(args);