tox: add generic xdist factor

Author: blueyed

tox.ini

@@ -20,18 +20,21 @@ envlist =
 
 [testenv]
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest --lsof {posargs}
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest {env:_PYTEST_TOX_ARGS:} {posargs}
     coverage: coverage combine
     coverage: coverage report
 passenv = USER USERNAME COVERAGE_* TRAVIS
 setenv =
+    _PYTEST_TOX_ARGS=--lsof
     # configuration if a user runs tox with a "coverage" factor, for example "tox -e py37-coverage"
     coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run -m
     coverage: _PYTEST_TOX_EXTRA_DEP=coverage-enable-subprocess
     coverage: COVERAGE_FILE={toxinidir}/.coverage
     coverage: COVERAGE_PROCESS_START={toxinidir}/.coveragerc
+    xdist: _PYTEST_TOX_ARGS={env:_PYTEST_TOX_ARGS:-n auto}
 extras = testing
 deps =
+    xdist: pytest-xdist>=1.13
     {env:_PYTEST_TOX_EXTRA_DEP:}
 
 [testenv:py27-subprocess]
@@ -49,22 +52,6 @@ basepython = python3
 deps = pre-commit>=1.11.0
 commands = pre-commit run --all-files --show-diff-on-failure
 
-[testenv:py27-xdist]
-extras = testing
-deps =
-    pytest-xdist>=1.13
-    {env:_PYTEST_TOX_EXTRA_DEP:}
-commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto {posargs}
-
-[testenv:py37-xdist]
-# NOTE: copied from above due to https://github.com/tox-dev/tox/issues/706.
-extras = testing
-deps =
-    pytest-xdist>=1.13
-    {env:_PYTEST_TOX_EXTRA_DEP:}
-commands = {[testenv:py27-xdist]commands}
-
 [testenv:py27-pexpect]
 platform = linux|darwin
 deps =