Add the test

Author: JakeWharton

rxandroid/src/test/java/io/reactivex/rxjava3/android/plugins/RxAndroidPluginsNoRobolectricTest.java

@@ -0,0 +1,22 @@
+package io.reactivex.rxjava3.android.plugins;
+
+import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers;
+import io.reactivex.rxjava3.android.testutil.EmptyScheduler;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.assertSame;
+
+public final class RxAndroidPluginsNoRobolectricTest {
+    @Before @After
+    public void setUpAndTearDown() {
+        RxAndroidPlugins.reset();
+    }
+    
+    @Test public void mainThreadSchedulerCanBeReplaced() {
+        EmptyScheduler emptyScheduler = new EmptyScheduler();
+        RxAndroidPlugins.setMainThreadSchedulerHandler(scheduler -> emptyScheduler);
+        assertSame(emptyScheduler, AndroidSchedulers.mainThread());
+    }
+}