[TECH DEBT] Use/wrap existing fake_service_instance

[waynew]

          That makes sense - I don't think it's worth blocking this PR over, but I'll go ahead and create an issue to track this. You can extend fixtures like:

@pytest.fixture
def custom_si(fake_service_instance):
    ...  # whatever changes you need to make
    yield fake_service_instance   # typically yield, unless the result is static, like `return 42` or a dictionary or something

I'll go ahead and create an issue from this to track efforts

Originally posted by @waynew in #333 (comment)

[waynew]

@zruzhinov