tox: split default posargs in multiple env vars

This is required for combining.

Use it for new lsof facor also.

tox.ini

@@ -21,12 +21,12 @@ envlist =
 
 [testenv]
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest {posargs:{env:_PYTEST_TOX_DEFAULT_POSARGS}}
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest {posargs:{env:_PYTEST_TOX_DEFAULT_POSARGS:}}
     coverage: coverage combine
     coverage: coverage report
 passenv = USER USERNAME COVERAGE_* TRAVIS PYTEST_ADDOPTS
 setenv =
-    _PYTEST_TOX_DEFAULT_POSARGS=--lsof
+    _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_PEXPECT:} {env:_PYTEST_TOX_POSARGS_XDIST:}
 
     # Configuration to run with coverage similar to Travis/Appveyor, e.g.
     # "tox -e py37-coverage".
@@ -35,10 +35,12 @@ setenv =
     coverage: COVERAGE_FILE={toxinidir}/.coverage
     coverage: COVERAGE_PROCESS_START={toxinidir}/.coveragerc
 
+    lsof: _PYTEST_TOX_POSARGS_LSOF=--lsof
+
     pexpect: _PYTEST_TOX_PLATFORM=linux|darwin
-    pexpect: _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_DEFAULT_POSARGS:testing/test_pdb.py testing/test_terminal.py testing/test_unittest.py}
+    pexpect: _PYTEST_TOX_POSARGS_PEXPECT=testing/test_pdb.py testing/test_terminal.py testing/test_unittest.py
 
-    xdist: _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_DEFAULT_POSARGS:-n auto}
+    xdist: _PYTEST_TOX_POSARGS_XDIST=-n auto
 extras = testing
 deps =
     pexpect: pexpect