Deflake should execute the Warmup function when Warmup group is started

The test is flaky as visible in both PRs and the periodic. The reason is
that it calls Start() which asynchronously starts a runnable and expects
said runnable to finish immediately after which may or may not work out
depending on how busy the box the test is run on is.

Use `synctest` instead, as that allows us to explicitly block until all
asynchronous operations that can finish are finished.

Author: alvaroaleman

pkg/manager/runnable_group_test.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"sync/atomic"
+	"testing"
+	"testing/synctest"
 	"time"
 
 	. "github.com/onsi/ginkgo/v2"
@@ -110,30 +112,6 @@ var _ = Describe("runnables", func() {
 		Expect(r.Others.startQueue).To(BeEmpty())
 	})
 
-	It("should execute the Warmup function when Warmup group is started", func(ctx SpecContext) {
-		var warmupExecuted atomic.Bool
-
-		warmupRunnable := newWarmupRunnableFunc(
-			func(c context.Context) error {
-				<-c.Done()
-				return nil
-			},
-			func(c context.Context) error {
-				warmupExecuted.Store(true)
-				return nil
-			},
-		)
-
-		r := newRunnables(defaultBaseContext, errCh)
-		Expect(r.Add(warmupRunnable)).To(Succeed())
-
-		// Start the Warmup group
-		Expect(r.Warmup.Start(ctx)).To(Succeed())
-
-		// Verify warmup function was called
-		Expect(warmupExecuted.Load()).To(BeTrue())
-	})
-
 	It("should propagate errors from Warmup function to error channel", func(ctx SpecContext) {
 		expectedErr := fmt.Errorf("expected warmup error")
 
@@ -384,3 +362,33 @@ func newLeaderElectionAndWarmupRunnable(
 func (r leaderElectionAndWarmupRunnable) NeedLeaderElection() bool {
 	return r.needLeaderElection
 }
+
+func TestWarmupFunctionIsExecutedWhenWarmupGroupIsStarted(t *testing.T) {
+	t.Parallel()
+	synctest.Test(t, func(t *testing.T) {
+		g := NewWithT(t)
+		var warmupExecuted atomic.Bool
+
+		warmupRunnable := newWarmupRunnableFunc(
+			func(c context.Context) error {
+				<-c.Done()
+				return nil
+			},
+			func(c context.Context) error {
+				warmupExecuted.Store(true)
+				return nil
+			},
+		)
+
+		r := newRunnables(defaultBaseContext, make(chan error))
+		g.Expect(r.Add(warmupRunnable)).To(Succeed())
+
+		// Start the Warmup group
+		g.Expect(r.Warmup.Start(t.Context())).To(Succeed())
+		synctest.Wait()
+
+		// Verify warmup function was called
+		g.Expect(warmupExecuted.Load()).To(BeTrue())
+		r.Warmup.StopAndWait(t.Context())
+	})
+}