* pmb/challenge/apk.py had to be renamed to pmb/challenge/apk_file.py,
so the "internal" functions of that file could be accessed, while
still providing the short notation pmb.challenge.apk().
* zap asks for each buildroot_* chroot, if you want to remove it, not
only for the one with the device arch
* add new pmb.chroot.tempfolder() function, that creates a temporary
folder, that belongs to "user" and deletes it, if it already exists.
this function gets used in a few challenge testcases.
For some reason, it was not possible to create the .tar.gz
archives with Python 3.4, that are used to simulate broken
or malicious apk downloads. I've rewritten the testcase,
so it creates the .tar.gz files inside the native Alpine
Linux chroot.
* automatically find the chroot binary on Debian, even if it is not
in the user's PATH
* don't use subprocess.run anymore (remove related testcase, that explicitly
checked for subprocess.run usage, and used recursive globbing, another
post 3.4 Python feature, for the checks. A similar case can be added in the
future, but right now it's more important to get Debian 3.4 working and all
PRs are reviewed anyway.)
* pytest fixtures: don't use the newer "yield" feature, as this is only
supported in a newer version of pytest, than provided on Debian Jessie
From manually testing, most stuff works in Debian Jessie. However, the
testsuite does not run through - creating an empty .tar.gz with Python
fails for some reason (this is done in test_apk_static.py).
