go/oasis-test-runner: Fix flaky key manager rotation failure test

In the key manager rotation failure test, start key managers at the beginning
of an epoch to ensure that both are added to the committee simultaneously.
If key managers are started randomly, there is a risk that only one will
be added to the committee, as the other may register in another epoch.

go/oasis-test-runner/scenario/e2e/runtime/keymanager_rotation_failure.go

@@ -76,13 +76,13 @@ func (sc *kmRotationFailureImpl) Fixture() (*oasis.NetworkFixture, error) {
 }
 
 func (sc *kmRotationFailureImpl) Run(ctx context.Context, _ *env.Env) error {
-	// Start the first two key managers.
+	// Start the first key manager.
 	if err := sc.Net.Start(); err != nil {
 		return err
 	}
 
 	for i := 0; i < 3; i++ {
-		// Start the third key manager.
+		// Start the second and the third key manager.
 		if err := sc.StartKeymanagers([]int{1, 2}); err != nil {
 			return err
 		}
@@ -122,6 +122,11 @@ func (sc *kmRotationFailureImpl) Run(ctx context.Context, _ *env.Env) error {
 		if err := sc.verifyMasterSecretRejections(ctx, 3); err != nil {
 			return err
 		}
+
+		// Make sure key managers register in the same epoch.
+		if err := sc.WaitEpochs(ctx, 1); err != nil {
+			return err
+		}
 	}
 
 	// Verify that master secret generation works after the third key manager is deregistered.