Debian Jessie/Python 3.4 support for the most part (#6)

* 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).

pmb/helpers/run.py

@@ -32,20 +32,22 @@ def core(args, cmd, log_message, log, return_stdout, check=True):
         ret = None
         if log:
             if return_stdout:
-                ret = subprocess.run(cmd, stdout=subprocess.PIPE,
-                                     check=check).stdout.decode('utf-8')
+                ret = subprocess.check_output(cmd).decode("utf-8")
                 args.logfd.write(ret)
             else:
-                subprocess.run(cmd, stdout=args.logfd, stderr=args.logfd,
-                               check=check)
+                subprocess.check_call(cmd, stdout=args.logfd,
+                                      stderr=args.logfd)
             args.logfd.flush()
         else:
             logging.debug("*** output passed to pmbootstrap stdout, not" +
                           " to this log ***")
-            subprocess.run(cmd, check=check)
+            subprocess.check_call(cmd)
 
     except subprocess.CalledProcessError as exc:
-        raise RuntimeError("Command failed: " + log_message) from exc
+        if check:
+            raise RuntimeError("Command failed: " + log_message) from exc
+        else:
+            pass
     return ret