change Xcode test run default to ignore xpass on Xunit output

The Green Dragon LLVM builders are starting to parse xunit output
on LLDB Xcode builders.  By default the XML formatter treats
xpass (unexpected successes) as failures.  The new flag added
ensures we simply ignore those for purposes of xUnit output.
LLDB is not currently XPASS clean.

llvm-svn: 257717