fix: replace t.Logf with slog in newMockCheck to fix data race

[lvlcn-t]

Motivation

The mock check's RunFunc and ShutdownFunc captured t *testing.T and called t.Logf from within a goroutine spawned by RegisterCheck. Since the parent subtest could finish before that goroutine executed, the Go race detector flagged a concurrent read/write on t's internal state.

Changes

• Removed the t *testing.T parameter from newMockCheck
• Removed t.Helper() call (no longer valid without t)
• Replaced t.Logf("Run called for check %s", name) with slog.InfoContext(ctx, "Run called", "check", name)
• Replaced t.Logf("Shutdown called for check %s", name) with slog.Info("Shutdown called", "check", name)
• Updated all 5 call sites to drop the t argument

For additional information look at the commits.

Tests done

• Unit tests succeeded (go test -race ./pkg/sparrow/... passes cleanly)
• E2E tests succeeded

TODO

• I've assigned this PR to myself
• I've labeled this PR correctly