Streamline the `if`/`else` cases in tests a bit.

[npalaska]

        if self.index_from_metadata:
            index = self.build_index_from_metadata()
        else:
            index = self.build_index(
                server_config,
                self.date_range(self.payload["start"], self.payload["end"]),
            )

This if/else is repeated several times, and that stands out in the pattern of your changes. It could really be refactored into a new superclass method, which would streamline the test cases a bit.

Originally posted by @dbutenhof in #2581 (comment)