Fix one set of tests, mark xfail for another test

[johnbaldwin]

Two PRs here

1. Set xfail to longstanding failing test

This is for code that is run to backfill data on new deployments.

Figures tests/test_backfill.py::test_backfill_monthly_metrics_for_site fails because the number of months expected to backfill do not match up

See: https://github.com/appsembler/figures/issues/354

2. Fixed the expected months back for site monthly metrics viewset tests. 6 months back are returned as expected, but the test case was expecting 7 months back