Fix tox.ini syntax for 4 for the main test environment

When parsing the configuration, tox would complain about the unfollowed
line continuation (which is what happens when the testenv was qualified
with neither full nor minimal).

Moving {posargs} to be unqualified allows the line continuation
character to always have something behind it.

tox.ini

 [tox]
 requires =
   tox>4
-envlist=black,flake8,mypy,py3-{minimal,full}
+envlist=black,flake8,mypy,py3-{minimal,full-cover}
 
 [testenv]
 extras =
   full: testing
   minimal: testing-minimal
 deps =
-  pytest-cov
+  cover: pytest-cov
 commands =
-  pytest \
-           --doctest-modules \
-  full:    --cov={envsitepackagesdir}/swh/model --cov-branch {posargs} \
-  full:    {envsitepackagesdir}/swh/model
-  minimal: {envsitepackagesdir}/swh/model/tests/test_cli.py -m 'not requires_optional_deps'
+  pytest --doctest-modules \
+  cover:   --cov={envsitepackagesdir}/swh/model --cov-branch \
+  full:    {envsitepackagesdir}/swh/model \
+  minimal: {envsitepackagesdir}/swh/model/tests/test_cli.py -m 'not requires_optional_deps' \
+           {posargs}
 
 [testenv:py3]
 skip_install = true
 deps = tox
 commands =
-  tox run -e py3-full -- {posargs}
+  tox run -e py3-full-cover -- {posargs}
   tox run -e py3-minimal -- {posargs}
 
 [testenv:black]