Add coverage of the holder class

Author: akarnokd

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());
+    }
+
+}