test/run-unit-tests, TEST-02: skip tests where the interpeter is not installed

test/run-unit-tests, TEST-02: skip tests where the interpeter is not installed

When the interpeter is missing, we get an exit code of 127. Let's treat those
tests as skipped too. If we could run the test far enough so that it could do
the check itself, it would return 77 anyway.

$ test/asdf; echo $?
exec: Failed to execute process 'test/asdf': The file specified the interpreter '/bin/asdf', which is not an executable command.
127
$ test/asdf; echo $?
/usr/bin/env: ‘/bin/asdf’: No such file or directory
127

This should resolve the problem that TEST-02 fails or Debian's 'unit-tests' fail
when python3 is not installed. Installing python3 via the mechanism that is
used to construct TEST images, i.e. the dracut dependency chasing scheme, would
be a lot of work for python with its modules in multiple locations and hundreds
of little files. So I think it OK to just skip the test there, and also in
other cases where python is not available.