[lit] Remove spurious `-` in invocation of lit in `shtest-xunit-output.py` test.
[lit] Remove spurious `-` in invocation of lit in
`shtest-xunit-output.py` test.

Although there is no `-` file Jeremy Morse has reported to me that it
causes problems in their setup because lit tries to find it and ends up
loading an out of tree lit configuration file.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@330728 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm