[SYNTH-22405]: Fix Synthetics Scheduler Flush by locking it (#41954)

### What does this PR do?

This PR fixes an issue in the Synthetics Scheduler where we weren't locking the flush function and therefore we had race conditions when updating the nextRun value for tests.

### Motivation

### Describe how you validated your changes

Local execution of the Agent.

### Additional Notes

Author: rafamelo12

comp/syntheticstestscheduler/impl/worker.go

@@ -59,16 +59,26 @@ func (s *syntheticsTestScheduler) flushLoop(ctx context.Context) {
 
 // flush enqueues tests whose nextRun is due.
 func (s *syntheticsTestScheduler) flush(flushTime time.Time) {
+	s.state.mu.Lock()
+	defer s.state.mu.Unlock()
+	var testsToRun []*runningTestState
 	for id, rt := range s.state.tests {
 		if flushTime.After(rt.nextRun) || flushTime.Equal(rt.nextRun) {
-			s.log.Debugf("enqueuing test %s", id)
-			s.syntheticsTestProcessingChan <- SyntheticsTestCtx{
-				nextRun: flushTime,
-				cfg:     rt.cfg,
-			}
-			s.updateTestState(rt)
+			s.log.Debugf("test %s is due for execution", id)
+			testsToRun = append(testsToRun, rt)
 		}
 	}
+
+	for _, rt := range testsToRun {
+		s.log.Debugf("enqueuing test %s", rt.cfg.PublicID)
+		s.syntheticsTestProcessingChan <- SyntheticsTestCtx{
+			nextRun: flushTime,
+			cfg:     rt.cfg,
+		}
+
+		rt.lastRun = rt.nextRun
+		rt.nextRun = rt.nextRun.Add(time.Duration(rt.cfg.Interval) * time.Second)
+	}
 }
 
 // runWorker is the main loop for a single worker.
@@ -212,14 +222,6 @@ type SyntheticsTestCtx struct {
 	cfg     common.SyntheticsTestConfig
 }
 
-// updateTestState updates lastRun and nextRun for a running test.
-func (s *syntheticsTestScheduler) updateTestState(rt *runningTestState) {
-	s.state.mu.Lock()
-	defer s.state.mu.Unlock()
-	rt.lastRun = rt.nextRun
-	rt.nextRun = rt.nextRun.Add(time.Duration(rt.cfg.Interval) * time.Second)
-}
-
 // sendSyntheticsTestResult marshals the workerResult and forwards it via the epForwarder.
 func (s *syntheticsTestScheduler) sendSyntheticsTestResult(w *workerResult) error {
 	res, err := s.networkPathToTestResult(w)