[lit] Add SKIPPED test result category

Track and print the number of skipped tests.  Skipped tests are tests
that should have been executed but weren't due to:
  * user interrupt [Ctrl+C]
  * --max-time (overall lit timeout)
  * --max-failures

This is part of a larger effort to ensure that all discovered tests are
properly accounted for.

Add test for overall lit timeout feature (`--max-time` option) to
observe skipped tests.  Extend test for `--max-failures` option.

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D77819