Fix flaky precommit java test on prism caused by PR 35202

.github/trigger_files/beam_PreCommit_Java.json

@@ -0,0 +1,4 @@
+{
+  "comment": "Modify this file in a trivial way to cause this test suite to run.",
+  "modification": 1
+}

sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go

@@ -161,6 +161,8 @@ type Config struct {
 	// MaxBundleSize caps the number of elements permitted in a bundle.
 	// 0 or less means this is ignored.
 	MaxBundleSize int
+	// Whether to use real-time clock as processing time
+	EnableRTC bool
 }
 
 // ElementManager handles elements, watermarks, and related errata to determine
@@ -2160,8 +2162,10 @@ func (em *ElementManager) ProcessingTimeNow() (ret mtime.Time) {
 	}
 
 	// "Test" mode -> advance to next processing time event if any, to allow execution.
-	if t, ok := em.processTimeEvents.Peek(); ok {
-		return t
+	if !em.config.EnableRTC {
+		if t, ok := em.processTimeEvents.Peek(); ok {
+			return t
+		}
 	}
 
 	// "Production" mode, always real time now.

sdks/go/pkg/beam/runners/prism/internal/execute.go

@@ -145,7 +145,20 @@ func executePipeline(ctx context.Context, wks map[string]*worker.W, j *jobservic
 	topo := prepro.preProcessGraph(comps, j)
 	ts := comps.GetTransforms()
 
-	em := engine.NewElementManager(engine.Config{})
+	config := engine.Config{}
+	m := j.PipelineOptions().AsMap()
+	if experimentsSlice, ok := m["beam:option:experiments:v1"].([]interface{}); ok {
+		for _, exp := range experimentsSlice {
+			if expStr, ok := exp.(string); ok {
+				if expStr == "prism_enable_rtc" {
+					config.EnableRTC = true
+					break // Found it, no need to check the rest of the slice
+				}
+			}
+		}
+	}
+
+	em := engine.NewElementManager(config)
 
 	// TODO move this loop and code into the preprocessor instead.
 	stages := map[string]*stage{}