[lit] Set --single-process for single tests and --threads=1

Summary:
Automatically upgrade debugging experience (single process, no thread
pool) when:
  1) we only run a single test
  2) user specifies `-j1`

Details:
Fix `--max-failures` in single process mode. Option did not have an
effect in single process mode.

Add display feedback for single process mode. Adapted test.

Improve argument checking (require positive integers).

`--single-process` is now essentially an alias for `-j1`. Should we
remove it?

Reviewers: rnk

Differential Revision: https://reviews.llvm.org/D58249

llvm-svn: 354068