Fix pytest on macOS in Travis (#5360)

* Add tools/pytest.sh

* pass TRAVIS through in tox.ini

* Use tools/pytest.sh to run pytest

* Add quiet to pytest.ini

* ignore pytest cache

tools/install_and_test.sh

@@ -23,5 +23,5 @@ for requirement in "$@" ; do
     # See https://travis-ci.org/certbot/certbot/jobs/308774157#L1333.
     pkg=$(echo "$pkg" | tr - _)
   fi
-  pytest --numprocesses auto --quiet --pyargs $pkg
+  "$(dirname $0)/pytest.sh" --pyargs $pkg
 done

tools/pytest.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# Runs pytest with the provided arguments, adding --numprocesses to the command
+# line. This argument is set to "auto" if the environmnent variable TRAVIS is
+# not set, otherwise, it is set to 2. This works around
+# https://github.com/pytest-dev/pytest-xdist/issues/9. Currently every Travis
+# environnment provides two cores. See
+# https://docs.travis-ci.com/user/reference/overview/#Virtualization-environments.
+
+if ${TRAVIS:-false}; then
+    NUMPROCESSES="2"
+else
+    NUMPROCESSES="auto"
+fi
+
+pytest --numprocesses "$NUMPROCESSES" "$@"