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

[seanh]

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.
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 (#9638) 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
    clear_search_index()

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.

[seanh]

I've removed the ELASTICSEARCH_INDEX and ELASTICSEARCH_URL constants as having to pass them around/import them into different files is a pain. Any file that wants these envvars can just read them directly from os.environ.

[seanh]

The code that created the ELASTICSEARCH_URL constant was defaulting it to localhost:9200 if the envvar didn't exist. I've moved that default value into tox.ini as we do for other envvar default values. This means the code can assume the envvar always exists.