Use assertNotNull instead of assertTrue(ref != null).

In response to review comments on 878d59e5.

Bug: 27940141
Change-Id: I9fb1ba5825d0305005f74424429626c59609f35a
diff --git a/test/vogar/target/junit4/MockitoFieldTest.java b/test/vogar/target/junit4/MockitoFieldTest.java
index 27cab49..e4f4bcc 100644
--- a/test/vogar/target/junit4/MockitoFieldTest.java
+++ b/test/vogar/target/junit4/MockitoFieldTest.java
@@ -18,7 +18,7 @@
 import org.junit.Test;
 import org.mockito.Mock;
 
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertNotNull;
 
 /**
  * Test that fields annotated with {@link Mock @Mock} are correctly initialized with a mock object.
@@ -30,6 +30,6 @@
 
     @Test
     public void test() {
-        assertTrue("Runnable field was not mocked", runnable != null);
+        assertNotNull("Runnable field was not mocked", runnable);
     }
 }