Only use function scoped fixtures.

I don't know why, but there is a particularly annoying edge case when
running the entire test suite, on Python 3.7, on Linux.

As far as I can tell, the server process for the code actions test
case never terminates. The test case works perfectly on all other
Python versions and platforms. It even works fine if you run the test
case on its own! But run the test case along with everything else, on
Linux using Python 3.7 - the server process hangs, hanging the rest of
the test suite with it.

Luckily, the fix appears to be simply not re-using the event loop
across test cases, thankfully we don't have any test cases that
require this yet.

tests/conftest.py

@@ -63,21 +63,21 @@ def client_server(request):
     client_server.stop()
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture()
 def server_dir():
     """Returns the directory where all the example language servers live"""
     path = pathlib.Path(__file__) / ".." / ".." / "examples" / "servers"
     return path.resolve()
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture()
 def workspace_dir():
     """Returns the directory containing the example workspace."""
     path = pathlib.Path(__file__) / ".." / ".." / "examples" / "workspace"
     return path.resolve()
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture()
 def event_loop():
     """Redefine `pytest-asyncio's default event_loop fixture to match the scope
     of our client fixture."""

tests/lsp/test_code_action.py

@@ -34,7 +34,7 @@
 from pygls.lsp.client import LanguageClient
 
 
-@pytest_asyncio.fixture(scope="module")
+@pytest_asyncio.fixture()
 async def client(server_dir):
     """Setup and teardown the client."""