Piuparts does not have `--list` (without further arguments) or `--info`
options, hence running the autopkgtest currently fails. Replace the two
broken stubs by two working tests:

 1. run `piuparts --version`
 2. create a minimal dummy binary package and run piuparts on it

Using a single-letter package name for the dummy package ensures that
there is no package with the same name in the archive -- according to
policy 5.6.1, all official package names must be at least two characters
long.

Move __pycache__ directory removal from debian/rules to the `clean`
Makefile target. The change should have no effect on the generated
packages; it just makes `make clean` more complete and debian/rules
simpler.

Instead of using a shell loop for passing the files to `rm`, also use
find's `-delete` option, avoiding any potential issues with weird
filenames.

Since the __pycache__ folders may contain .pyc files, make sure these
are all deleted before.

Should fix:

Traceback (most recent call last):
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1911, in <module>
    main()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1851, in main
    section.generate_output(output_directory, section_names, problem_list, web_host)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1660, in generate_output
    self.generate_html()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1587, in generate_html
    failures = dwke_get_failures(self._binary_db, self._problem_list)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1761, in dwke_get_failures
    add_cnt = make_kprs(logdict, kprdict, problem_list)
  File "/srv/piuparts.debian.org/lib/python3/dist-packages/piupartslib/dwke.py", line 252, in make_kprs
    logbody = lb.read()
  File "/usr/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf1 in position 26184: invalid continuation byte

Signed-off-by: Holger Levsen <holger@layer-acht.org>