tests: Use pointers in calls to g_atomic_pointer_compare_and_exchange()
Don’t pass integers; it’s not type-safe. The macro version of
g_atomic_pointer_compare_and_exchange()
used to erroneously accept
integers, but they would have the wrong width on some platforms.
Changes suggested by Ting-Wei Lan and Simon McVittie.
Fixes: #1940 (closed)