Drop a questionable test from the refstring suite

Testing that an interned string is released once its last reference is dropped is not possible without a hook into the interning machinery; just checking that the returned pointer for the same string is going to be different after the last release() is not guaranteed to work, as the systema allocator is perfectly within its rights to recycle pointers, as long as they are unique while valid.

Closes #1467 (closed)