Make function-level setup fixture clear cache

This means that anyone adding a new test to this file doesn’t have to
remember to clear the cache in their test, or forget to and have a
hard-to-debug test failure.

Using `setup_function` means we don’t have to convert this module into
using class-based tests.

tests/app/delivery/test_send_to_providers.py

@@ -34,6 +34,12 @@ from tests.app.db import (
 )
 
 
+def setup_function(_function):
+    # pytest will run this function before each test. It makes sure the
+    # state of the cache is not shared between tests.
+    send_to_providers.provider_cache.clear()
+
+
 def test_provider_to_use_should_return_random_provider(mocker, notify_db_session):
     mmg = get_provider_details_by_identifier('mmg')
     firetext = get_provider_details_by_identifier('firetext')
@@ -53,8 +59,6 @@ def test_provider_to_use_should_cache_repeated_calls(mocker, notify_db_session):
         wraps=send_to_providers.random.choices,
     )
 
-    send_to_providers.provider_cache.clear()
-
     results = [
         send_to_providers.provider_to_use('sms', international=False)
         for _ in range(10)
@@ -90,8 +94,6 @@ def test_provider_to_use_raises_if_no_active_providers(mocker, restore_provider_
     mmg = get_provider_details_by_identifier('mmg')
     mmg.active = False
 
-    send_to_providers.provider_cache.clear()
-
     with pytest.raises(Exception):
         send_to_providers.provider_to_use('sms', international=True)
 
@@ -643,7 +645,6 @@ def test_should_send_sms_to_international_providers(
         international=True,
         reply_to_text=sample_template.service.get_default_sms_sender()
     )
-    send_to_providers.provider_cache.clear()
     send_to_providers.send_sms_to_provider(
         notification_uk
     )
@@ -694,7 +695,6 @@ def test_should_handle_sms_sender_and_prefix_message(
     template = create_template(service, content='bar')
     notification = create_notification(template, reply_to_text=sms_sender)
 
-    send_to_providers.provider_cache.clear()
     send_to_providers.send_sms_to_provider(notification)
 
     mmg_client.send_sms.assert_called_once_with(