Add posargs when running pytests

This small change in tox.ini allow giving pytests extra arguments, for
e.g.
tox -e pypy-39-unit -- -k test_heartbeat_check_rate_ -s

which allow running only part of the tests (useful for debug purpose).

Signed-off-by: Arnaud Morin <arnaud.morin@ovhcloud.com>
Change-Id: I535acdb918c6caaae6e6ea4596437076a38f7a59

tox.ini

@@ -18,7 +18,7 @@ deps=
 sitepackages = False
 recreate = False
 commands =
-    unit: py.test -xv --cov=amqp --cov-report=xml --no-cov-on-fail t/unit
+    unit: py.test -xv --cov=amqp --cov-report=xml --no-cov-on-fail t/unit {posargs}
     integration: py.test -xv -E rabbitmq t/integration {posargs:-n2}
 basepython =
     flake8,apicheck,linkcheck,pydocstyle: python3.8