xUnit test formatter: add options for ignoring skipped tests

Skipped tests can be dropped from xUnit reports if either
the name or the skip reason matches one of a given set of
regular expression patterns (via re.search(), not re.match()).
New formatter option for the xunit formatter:
--ignore-skip-matching-reason and
--ignore-skip-matching-name

Both are results-formatter options.

llvm-svn: 248247