In parallel mode, tox marks skipped interpreters as succeded.

[awecx]

Say I have no Python3.3 interpreter on my machine.
The following tox.ini:

[tox]
envlist = py33
skip_missing_interpreters = True

would of course yield:

python3 -m tox
GLOB sdist-make: /xxx/yyy/setup.py
py33 recreate: /xxx/yyy/.tox/py33
SKIPPED: InterpreterNotFound: python3.3
__________________________________________________________________ summary ___________________________________________________________________
SKIPPED:  py33: InterpreterNotFound: python3.3

but when run in parallel, we get:

python3 -m tox -p auto
GLOB sdist-make: /xxx/yyy/setup.py
✔ OK py33 in 0.22 seconds
__________________________________________________________________ summary ___________________________________________________________________
  py33: commands succeeded
  congratulations :)

That could make a user to think that his tests are green while they simply … did not run.
Tested with tox 3.14.3.

[rpkilby]

Duplicate of #1274.