use more modern test assertions in some places

[majewsky]

I looked at every t.Fatal(f) and t.Error(f) to check if we can replace those with more appropriate assertions from go-bits/assert or go-bits/must. There are probably other places where those functions could be used, but I decided to limit myself to however much I could get done before the end of the day.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/limes/internal/api	78.07% (ø)
github.com/sapcc/limes/internal/collector	82.40% (ø)
github.com/sapcc/limes/internal/core	78.35% (ø)
github.com/sapcc/limes/internal/datamodel	85.92% (ø)
github.com/sapcc/limes/internal/test	92.07% (+1.96%)	👍
github.com/sapcc/limes/internal/util	83.52% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/limes/internal/test/setup.go	91.87% (+2.56%)	738 (-48)	678 (-24)	60 (-24)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/sapcc/limes/internal/api/api_test.go
• github.com/sapcc/limes/internal/api/commitment_test.go
• github.com/sapcc/limes/internal/collector/capacity_scrape_test.go
• github.com/sapcc/limes/internal/collector/commitment_cleanup_test.go
• github.com/sapcc/limes/internal/collector/consistency_test.go
• github.com/sapcc/limes/internal/collector/expiring_commitments_test.go
• github.com/sapcc/limes/internal/collector/keystone_test.go
• github.com/sapcc/limes/internal/collector/mail_delivery_test.go
• github.com/sapcc/limes/internal/collector/quota_overrides_test.go
• github.com/sapcc/limes/internal/collector/scrape_test.go
• github.com/sapcc/limes/internal/core/config_test.go
• github.com/sapcc/limes/internal/datamodel/allocation_stats_test.go
• github.com/sapcc/limes/internal/datamodel/apply_computed_project_quota_test.go
• github.com/sapcc/limes/internal/datamodel/quota_overrides_test.go
• github.com/sapcc/limes/internal/util/timeseries_test.go