[lit] Include unexecuted tests in xUnit report

Pass in all discovered tests to report generators.

The XunitReport generator now creates testcase items for unexecuted
tests and documents why they have been skipped.  This makes it easier
to compare test runs with different filters or configurations, or across
platforms.

I don't know who is using the JsonReport generator and what the
expectations there are (it doesn't have tests), so decided to preserve
the old behavior by filtering out the unexecuted tests.

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D81316