Automatically set environment vars before tests

Sometimes you just wanna run some tests directly using the `pytest`
command. But you’re in a new shell, and have forgotten to do
`source environment_test.sh`. The screen fills with red, and your day
just got a little bit worse.

This commit will stop this from ever happening again, by making the
setting environment variables part of running Pytest. It does this with
a plugin called pytest-env[1].

pytest.ini is the standard way of configuring pytest. Creating this file
where it didn’t exist before changes the behaviour of pytest, in that
it will now look for tests in the same directory as the file, rather
than defaulting to the `tests/` directory. So we also have to explicitly
configure pytest[2] to tell it that it should only look in this
directory. Otherwise it gets lost in the weeds of `node_modules`.

1. https://github.com/MobileDynasty/pytest-env
2. https://docs.pytest.org/en/latest/customize.html#confval-testpaths

pytest.ini

@@ -0,0 +1,11 @@
+[pytest]
+testpaths = tests
+env =
+    NOTIFY_ENVIRONMENT=test
+    ADMIN_CLIENT_SECRET=dev-notify-secret-key
+    API_HOST_NAME=test
+    DANGEROUS_SALT=dev-notify-salt
+    SECRET_KEY=dev-notify-secret-key
+    DESKPRO_API_HOST=test
+    DESKPRO_API_KEY=test
+    STATSD_PREFIX=stats-prefix