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
18df607dbe