Fix arithmetic error in multi_container_test.

We add an additional (2^3)-1=7 processes, but the code was only waiting for 3.

I switched back to Math.Pow format to make the arithmetic easier to inspect.

PiperOrigin-RevId: 215588140
Change-Id: Iccad4d6f977c1bfc5c4b08d3493afe553fe25733

runsc/container/multi_container_test.go

@@ -568,7 +568,8 @@ func TestMultiContainerKillAll(t *testing.T) {
 		if _, err := containers[1].Execute(args); err != nil {
 			t.Fatalf("error exec'ing: %v", err)
 		}
-		procCount += 3
+		// Wait for these new processes to start.
+		procCount += int(math.Pow(2, 3) - 1)
 		if err := waitForProcessCount(containers[1], procCount); err != nil {
 			t.Fatal(err)
 		}