[Utils] Ensure that empty.txt is always empty (#157576)
Previously this test case would `touch %S/empty.txt` to create and empty
file. The test case then copies contents to that file, so if run a
second time the `touch` command would not create an empty file.
`llvm/utils/lit/tests/diff-test-update.py` also no longer matches against file
paths other than the final bit of the file name. This prevents test failures
on Windows due to different path separators.
