MainLoop: Add unit tests

Summary:
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

llvm-svn: 302133