Detect the case that one file has a SELinux label but the other
file does not have any label at all. This was currently not
possible to detect because both calls to get the labels were
wrapped in one try-except block and any failure in one of the
two calls, like a missing label, would lead to an early return
of the function, with a success value.
Move the `tree-diff` tool into ./tools, which is our new place for tools
used by the test-suite or during development.
The only hard-coded user is the TestBase, so fix its path to the tool
so the test-suite will continue to find it.
