[lit] Fix lldbtest format
[lit] Fix lldbtest format

The lldbtest format was incorrectly detecting XFAIL as FAIL because it
was looking for the `FAIL:` substring in the dotest output.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@351497 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb