chore: use testify's require in tests - #2792

This PR standardizes the Grafana alertcheck test suite on testify/require for fail-fast assertions, adds the testify module dependency, and introduces a recorder-mode “fail fast” in internal/gate/check.go when from is before the recorded header’s StartedAt.

Changes:

• Replace hand-rolled t.Fatal/t.Error patterns across tests with require.* assertions.
• Add github.com/stretchr/testify (and indirect YAML dep) to go.mod/go.sum.
• Add a recorder-mode early exit in check.go for the statically-detectable from < StartedAt bound violation.

Suppressed comments (1)

grafana-alertcheck/internal/gate/check_test.go:895

• This responder can be invoked from goroutines via observeAll during the drain wait. require.Fail triggers FailNow from a worker goroutine, which doesn't reliably stop the test. Prefer returning an error (the caller path already makes the test fail by violating the require.NoError assertion).

		src := newCheckSource(func(title string, _ int) (Observation, error) {
			require.Fail(t, fmt.Sprintf("the drain wait polled skipped rule %q", title))
			return Observation{}, errors.New("unexpected poll")
		})

• Files reviewed: 20/21 changed files
• Comments generated: 2
• Review effort level: Lite