Add test asserting old metric name is NOT present in output

[mvillmow]

The current test only checks that homeric_exporter_scrape_timestamp_seconds is present. It does not verify that the old name homeric_exporter_scrape_timestamp (without _seconds) is absent, which would catch a regression where both names are emitted. Add a assertNotIn('homeric_exporter_scrape_timestamp{', self.output) guard that distinguishes the bare name from the suffixed one.

Follow-up from #156