Flaky TestGameServerUnhealthyAfterReadyCrash (#2302)

I noticed in a few flaky end-to-end tests, I kept seeing this in the
logs:

```
time="2021-10-08 19:02:01.419" level=info msg="sent UDP packet" address="35.247.94.25:7682" test=TestGameServerUnhealthyAfterReadyCrash
```

Over and over again, and also noting that it was happening _after_ the
e2e test had completed.

See:
https://console.cloud.google.com/cloud-build/builds/9ca5715a-443c-4693-bbd5-2879e61f2aaa;step=21?project=agones-images
https://console.cloud.google.com/cloud-build/builds/84cb8db2-1a11-4db3-a9e1-d9d51b9baf14;step=21?project=agones-images

My theory: The go routine had nothing in it that forced it to stop once
the test was complete - so depending on order of tests, it might keep
continuing for a while, while other tests ran.

If it did that, and a GameServer spun up on the same node and port as
the originally crashed GameServer, it would crash it - likely breaking
whatever test it ran into!

Work on #2296

Co-authored-by: Robert Bailey <[email protected]>

test/e2e/gameserver_test.go

@@ -22,6 +22,7 @@ import (
 	"net"
 	"os/exec"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -385,8 +386,16 @@ func TestGameServerUnhealthyAfterReadyCrash(t *testing.T) {
 
 	// keep crashing, until we move to Unhealthy. Solves potential issues with controller Informer cache
 	// race conditions in which it has yet to see a GameServer is Ready before the crash.
+	var stop int32 = 0
+	defer func() {
+		atomic.StoreInt32(&stop, 1)
+	}()
 	go func() {
 		for {
+			if atomic.LoadInt32(&stop) > 0 {
+				l.Info("UDP Crash stop signal received. Stopping.")
+				return
+			}
 			conn, err := net.Dial("udp", address)
 			assert.NoError(t, err)
 			defer conn.Close() // nolint: errcheck