[lit] Don't include tests skipped due to sharding in reports

When running multiple shards, don't include skipped tests in the xunit
output since merging the files will result in duplicates.
In our CHERI Jenkins CI, I configured the libc++ tests to run using sharding
(since we are testing using a single-CPU QEMU). We then merge the generated
XUnit xml files to produce a final result, but if the individual XMLs
report tests excluded due to sharding each test is included N times in the
final result. This also makes it difficult to find the tests that were
skipped due to missing REQUIRES: etc.

Reviewed By: yln

Differential Revision: https://reviews.llvm.org/D84235