Move tests/common/fixtures/* into conftest.py

Move all the pytest fixtures defined in `tests/common/fixtures/*.py`
files into `conftest.py` files.

Context
-------

pytest provides a method of organising fixtures by putting them into
hierarchical/local `conftest.py` files:

    tests/
      conftest.py  # Fixtures defined in this file are available to all tests.
      unit/
        conftest.py  # Available to all unit tests.
      functional/
        conftest.py  # Available to all functests.
        foo/
          conftest.py  # Available to all tests in foo/ and below.

pytest automatically discovers and registers fixtures defined in
`conftest.py` files and makes them available to other files (both test
files and lower `conftest.py` files) for fixture injection _without
those files having to import the fixtures_.

See:

https://docs.pytest.org/en/stable/reference/fixtures.html#conftest-py-sharing-fixtures-across-multiple-files

A downside of this is that `conftest.py` files could get quite large but
it shouldn't get too bad: you shouldn't have too many shared fixtures
anyway.

At Hypothesis, [we thought we knew better](#5002).
We allowed this downside to tempt us to create
`tests/common/fixtures/*.py` files, allowing us to split our fixtures up
into multiple separate files. Our `conftest.py` files then needed to
import these fixtures from `tests.common.fixtures` so that pytest would
register them.

Problem
-------

Ah, hubris. In an upcoming future PR I need to have a common `es_client`
fixture that's shared between the unittests and the functests, and I
need to override that fixture to add some custom behaviour for the
unittests only. Here's how you'd normally do it with `conftest.py`
files, this works:

    # tests/conftest.py

    @pytest.fixture
    def es_client():
        client = _es_client()
        yield client
        client.close()

    # tests/unit/conftest.py

    @pytest.fixture
    def es_client(es_client):
        yield es_client

`tests/conftest.py` defines an `es_client` fixture that's available to
all test files and `conftest.py` files in the `tests/` directory, both
unittests and functests.

`tests/unit/conftest.py` then overrides the `es_client` fixture with its
own `es_client` fixture that depends on the higher-level `es_client`
fixture and adds some additional teardown.

This is normal pytest fixture overriding.

With our `tests/common/fixtures/` directory this doesn't work:

    # tests/common/fixtures/elasticsearch.py

    @pytest.fixture
    def es_client():
        client = _es_client()
        yield client
        client.close()

    # tests/unit/conftest.py

    from tests.common.fixtures.elasticsearch import es_client

    @pytest.fixture
    def es_client(es_client):
        yield es_client
        clear_search_index()

What happens is:

1. `tests/unit/conftest.py` has to import the `es_client` fixture from
   `tests/common/fixtures/elasticsearch.py`, otherwise pytest won't
   discover the fixture.
2. When `tests/unit/conftest.py` then defines its own `es_client`
   function that overrides the name `es_client` in the module's
   namespace, so now pytest will _not_ discover the original `es_client`
   function that was imported from the common directory.
3. When pytest sees an `es_client` fixture that depends on a fixture
   named `es_client`, it thinks the fixture is trying to depend on
   itself and crashes with a circular fixture dependency error.

Solution
--------

Get rid of the `tests/common/fixtures/` directory. Just do the normal
pytest thing and move all these fixtures into `tests/conftest.py`. When
any other files (including lower `conftest.py` files) want to use any of
these common fixtures they can just do so via pytest fixture injection
without needing to import anything.

There were some fixtures in `tests/common/fixtures/services.py` that
were actually only used in the unittests. These have been moved into
`tests/unit/h/conftest.py`.

It *is* possible to have our cake and eat it here: you can put your
fixtures in separate files (thus avoiding have them all in one large
file) and then import them all into the top-level `tests/conftest.py`
file and all other files use the fixtures via injection. But I think
it's simpler just to put shared fixtures in `conftest.py` files.

Author: seanh

.cookiecutter/includes/tox/passenv

@@ -14,5 +14,4 @@ dev: ORCID_HOST
 dev: ORCID_CLIENT_ID
 dev: ORCID_CLIENT_SECRET
 {tests,functests}: DATABASE_URL
-{tests,functests}: ELASTICSEARCH_URL
 functests: BROKER_URL

.cookiecutter/includes/tox/setenv

@@ -11,6 +11,7 @@ dev: H_API_AUTH_COOKIE_SECRET_KEY = {env:H_API_AUTH_COOKIE_SECRET_KEY:"dev_h_api
 dev: H_API_AUTH_COOKIE_SALT = {env:H_API_AUTH_COOKIE_SALT:"dev_h_api_auth_cookie_salt"}
 dev: REPLICA_DATABASE_URL = {env:DATABASE_URL:postgresql://postgres@localhost/postgres}
 dev: MAILCHIMP_USER_ACTIONS_SUBACCOUNT = {env:MAILCHIMP_USER_ACTIONS_SUBACCOUNT:devdata}
+tests,functests: ELASTICSEARCH_URL = {env:ELASTICSEARCH_URL:http://localhost:9200}
 tests: ELASTICSEARCH_INDEX = {env:ELASTICSEARCH_INDEX:hypothesis-tests}
 functests: ELASTICSEARCH_INDEX = {env:ELASTICSEARCH_INDEX:hypothesis-functests}
 {tests,functests}: AUTHORITY = {env:AUTHORITY:example.com}