at_exit test function fails

[brson]

As reported in #4656 the core test for at_exit seems to be failing sometimes. It's failing because the at_exit registration function expects to be passed the same pointer on each call but apparently isn't. I can imagine this could be because of the way coretest works, so coretest ends up using a different function than core. I can't though explain why this isn't showing up on the bots or in my tests.

[bstrie]

@brson is this still relevant? I'm not even sure which of the two at_exit tests this is referring to.

[alexcrichton]

There appears to no longer be any at_exit function in the new runtime, except for a possible TLS destructor which will be removed in #8447