ref: fix mock_redis_buffer (#82560)

when implemented as a `contextmanager` it cannot decorate a class
leading to
tests/sentry/rules/processing/test_processor.py::RuleProcessorTest never
running

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src/sentry/testutils/helpers/redis.py

@@ -9,11 +9,8 @@ from sentry.buffer.redis import RedisBuffer
 from sentry.testutils.helpers import override_options
 
 
-@contextmanager
 def mock_redis_buffer():
-    buffer = RedisBuffer()
-    with patch("sentry.buffer.backend", new=buffer):
-        yield buffer
+    return patch("sentry.buffer.backend", new=RedisBuffer())
 
 
 @contextmanager