Per-test thread overhead added in #8531 hits fd limit on OS X

[lilyball]

The fd limit issue on OS X rises again. With #8531 merged in, compiletest hits the fd limit. The workaround we have for it is only triggered when running the scheduler tests. Looks like compiletest needs it too.

[lilyball]

Referenced workaround is introduced in 2001cc0

[brson]

#8531 is reverted by #8645

[alexcrichton]

This is no longer an issue. The compiletest runner specifically raises the limit and so do the other portions of the infrastructure which ran into this limit.