etcd-runner: integrate etcd runner in to etcd tester

etcd tester runs etcd runner as a separate binary.
it signals sigstop to the runner when tester wants to stop stressing.
it signals sigcont to the runner when tester wants to start stressing.
when tester needs to clean up, it signals sigint to runner.

FIXES #7026

tools/functional-tester/etcd-tester/v2_stresser.go

@@ -93,11 +93,15 @@ func (s *v2Stresser) run(ctx context.Context, kv clientV2.KeysAPI) {
 	}
 }
 
-func (s *v2Stresser) Cancel() {
+func (s *v2Stresser) Pause() {
 	s.cancel()
 	s.wg.Wait()
 }
 
+func (s *v2Stresser) Close() {
+	s.Pause()
+}
+
 func (s *v2Stresser) ModifiedKeys() int64 {
 	return atomic.LoadInt64(&s.atomicModifiedKey)
 }