MainLoop: Add unit tests
This adds a couple of unit tests to the MainLoop class. To get the
kqueue based version of the signal handling passing, I needed to
modify the implementation a bit to make the queue object persistent.
Otherwise, only the signals which are send during the Run call would get
processed, which did not match the ppoll behaviour.
I also took the opportunity to remove the ForEach template functions and
replace them with something more reasonable.
Reviewers: beanz, eugene
Subscribers: lldb-commits, mgorny
Differential Revision: https://reviews.llvm.org/D32753