store: A1 — check rows.Err() + Scan err in migrate_test

Code-quality nits flagged in review of 2692c66. Mirrors the existing
pattern in host_credentials_test.go.

internal/store/migrate_test.go

@@ -24,9 +24,14 @@ func TestMigration0013AlertsLastSeen(t *testing.T) {
 	cols := map[string]bool{}
 	for rows.Next() {
 		var n string
-		_ = rows.Scan(&n)
+		if err := rows.Scan(&n); err != nil {
+			t.Fatalf("scan: %v", err)
+		}
 		cols[n] = true
 	}
+	if err := rows.Err(); err != nil {
+		t.Fatalf("rows iter: %v", err)
+	}
 	if !cols["last_seen_at"] {
 		t.Fatalf("alerts.last_seen_at not present after migration; cols=%v", cols)
 	}