Give a fast-renewing test lease a serving controller's time to renew, and prove the minted hash's cost

A renewal timed out at the renew interval, so the lease test's 1 s renewals
could lose the lease on a slow build machine (novox/hq issue 591, review of #279).

internal/inventory/bususers_test.go

@@ -164,7 +164,11 @@ func TestABusPasswordIsHashedAtTheDefaultCostOutsideATest(t *testing.T) {
 	was := busPasswordCostForTest.Load()
 	t.Cleanup(func() { busPasswordCostForTest.Store(was) })
 	busPasswordCostForTest.Store(0)
-	if got := busPasswordCost(); got != bcrypt.DefaultCost {
-		t.Fatalf("a bus password is hashed at cost %d, not bcrypt's default %d", got, bcrypt.DefaultCost)
+	hash, err := hashBusPassword("a password")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, err := bcrypt.Cost(hash); err != nil || got != bcrypt.DefaultCost {
+		t.Fatalf("a bus password is hashed at cost %d (%v), not bcrypt's default %d", got, err, bcrypt.DefaultCost)
 	}
 }