This is needed because on a mounted `bootc` container `setfiles`
without excluding `/sysroot` will create many warnings like:
```
setfiles: conflicting specifications for /run/osbuild/tree/sysroot/ostree/repo/objects/00/0ef9ada2ee87792e8ba21afd65aa00d79a1253018832652b8694862fb80e84.file and /run/osbuild/tree/usr/lib/firmware/cirrus/cs35l41-dsp1-spk-prot-103c8b8f-r1.bin.xz, using system_u:object_r:lib_t:s0.
```
but simply excluding this dir fixes them.
This is basically a re-implementation of `setfilecon(3)` minus the
translation of human readable context to raw context. Add test for
the new function.
Move the 'test_util_selinux.py' test into the module-unittest
subdirectory.
Drop the '__main__' hookup while at it. `python -m unittest --help`
explains how you can run individual tests.
