pytest-xdist: adjust parallelism, fixes #2053

it's either auto or env var XDISTN value.

tox.ini

@@ -12,7 +12,7 @@ deps =
      -rrequirements.d/development.txt
      -rrequirements.d/attic.txt
      -rrequirements.d/fuse.txt
-commands = py.test -n 8 -rs --cov=borg --cov-config=../.coveragerc --benchmark-skip --pyargs {posargs:borg.testsuite}
+commands = py.test -n {env:XDISTN:auto} -rs --cov=borg --cov-config=../.coveragerc --benchmark-skip --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *