[lit] Add EXCLUDED test result category

Track and print the number of tests that were discovered but not
executed due to test selection options:
  * --filter (regex filter)
  * --max-tests (limits number of tests)
  * sharding feature

With this change all discovered tests are accounted for: every
discovered test is included in one of the counts printed in the summary.

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D78078