3.x: Fix ExecutorScheduler initializing Schedulers prematurely (#7323)

* 3.x: Fix ExecutorScheduler initializing Schedulers prematurely

* Add coverage of the holder class

src/main/java/io/reactivex/rxjava3/internal/schedulers/ExecutorScheduler.java

@@ -38,7 +38,9 @@ public final class ExecutorScheduler extends Scheduler {
     @NonNull
     final Executor executor;
 
-    static final Scheduler HELPER = Schedulers.single();
+    static final class SingleHolder {
+        static final Scheduler HELPER = Schedulers.single();
+    }
 
     public ExecutorScheduler(@NonNull Executor executor, boolean interruptibleWorker, boolean fair) {
         this.executor = executor;
@@ -97,7 +99,7 @@ public Disposable scheduleDirect(@NonNull Runnable run, final long delay, final
 
         final DelayedRunnable dr = new DelayedRunnable(decoratedRun);
 
-        Disposable delayed = HELPER.scheduleDirect(new DelayedDispose(dr), delay, unit);
+        Disposable delayed = SingleHolder.HELPER.scheduleDirect(new DelayedDispose(dr), delay, unit);
 
         dr.timed.replace(delayed);
 
@@ -215,7 +217,7 @@ public Disposable schedule(@NonNull Runnable run, long delay, @NonNull TimeUnit
                     return EmptyDisposable.INSTANCE;
                 }
             } else {
-                final Disposable d = HELPER.scheduleDirect(sr, delay, unit);
+                final Disposable d = SingleHolder.HELPER.scheduleDirect(sr, delay, unit);
                 sr.setFuture(new DisposeOnCancel(d));
             }
 

src/test/java/io/reactivex/rxjava3/internal/schedulers/ExecutorSchedulerInternalTest.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.rxjava3.internal.schedulers;
+
+import static org.junit.Assert.assertNotNull;
+
+import org.junit.Test;
+
+public class ExecutorSchedulerInternalTest {
+
+    @Test
+    public void helperHolder() {
+        assertNotNull(new ExecutorScheduler.SingleHolder());
+    }
+
+}