Add more common errors to JUnit4 documentation

testing/android/docs/junit4.md

 # JUnit 4 Migration
 
 As of Android 24 (N), JUnit3 style javatests have been deprecated for the new
 JUnit4-based [Android Testing Support Library][1].
 We are in the process of changing all instrumentation tests in chromium to
 JUnit4 style. This doc explains the differences between JUnit3 and JUnit4
 instrumentation tests and how to write or convert them.
 
 [TOC]
 
@@ skipping 128 matching lines @@
                 // 2: Code here runs before @Before method
                 base.evaluate()
                 // 3: Code here runs after @After method
             }
         }
     }
 }
 ```
 
 
-## Caveats
+## Common Errors
 
 1.  Instrumentation tests that rely on test thread to have message handler
     will not work. For example error message:
 
         java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
 
-    Please utilize `@UiThreadTest` or
-    `ActivityTestRule.runOnUiThread(Runnable r)` to refactor these tests.
-    For more, check this [GitHub issue][6]
+    or
 
-1.  Use `@UiThreadTest` with caution. Currently,
-    **@UiThreadTest is only effective when UiThreadTestRule or
-    ActivityTestRule is declared** in the test class. Please use
-    `android.support.test.annotation.UiThreadTest`, not
-    `android.test.UiThreadTest`. When using @UiThreadTest, **it would cause
-    `setUp` and `tearDown` to run in Ui Thread** as well. Avoid that by simply
-    calling [`runOnUiThread`][9] or [`runOnMainSync`][10] with a Runnable.
+        java.lang.IllegalStateException: The current thread must have a looper!
+
+    Please utilize `ActivityTestRule.runOnUiThread(Runnable r)` to refactor
+    these tests. For more, check this [GitHub issue][6]
+
+1.  Use `@UiThreadTest` with caution!!
+    -   Currently, **@UiThreadTest is only effective when UiThreadTestRule or
+        ActivityTestRule is declared** in the test class.
+    -   Please use **`android.support.test.annotation.UiThreadTest`, NOT
+        `android.test.UiThreadTest`**.
+    -   When using @UiThreadTest, **it would cause `setUp` and `tearDown` to
+        run in Ui Thread** as well. Avoid that by calling [`runOnUiThread`][9]
+        or [`runOnMainSync`][10] with a Runnable.
     
     ```java
     // Wrong test
     public class Test {
         @Rule
         public ActivityTestRule<MyActivity> mRule = new ActivityTestRule<>(
                 MyActivity.class>
 
         @Before
         public void setUp() {
@@ skipping 27 matching lines @@
             mRule.runOnUiThread(new Runnable() {
                 @Override
                 public void run() {
                     actionThatNeedsUiThread();
                 }
             });
         }
     }
     ```
 
-
 1.  `assertEquals(float a, float b)` and `assertEquals(double a, double b)` are
     deprecated in JUnit4's Assert class. **Despite only generating a warning at
     build time, they fail at runtime.** Please use
     `Assert.assertEquals(float a, float b, float delta)`
 
+1.  Errorprone expects all public methods starting with `test...` to be
+    annotated with `@Test`. Failure to meet that expectation will cause
+    errorprone to fail with something like this: 
+
+        [JUnit4TestNotRun] Test method will not be run; please add @Test annotation
+
+    In particular, you may see this when attempting to disable tests. In that
+    case, the test should be annotated with both @DisabledTest and @Test.
 
 ## Common questions
 
 -   Q: Are `@Test` and `@LargeTest/@MediumTest/@SmallTest` annotation both
     necessary?
     -   A: Yes, both are required for now. We plan to refactor this in the
         future.
 -   Q: Isn't the inheritance of the Test classes just migrated to inheritance
     of TestRules?
     -   A: Yes. During the migration, we plan to maintain a 1:1 mapping between
@@ skipping 15 matching lines @@
 [1]: https://developer.android.com/topic/libraries/testing-support-library/index.html
 [2]: https://cs.chromium.org/chromium/src/android_webview/tools/system_webview_shell/layout_tests/AndroidManifest.xml?l=36
 [3]: http://junit.org/junit4/javadoc/4.12/org/junit/rules/TestRule.html
 [4]: https://developer.android.com/reference/android/support/test/rule/ActivityTestRule.html
 [5]: https://github.com/yoland68/chromium-junit-auto-migrate
 [6]: http://github.com/skyisle/android-test-kit/issues/121
 [7]: https://bugs.chromium.org/p/chromium/issues/detail?id=640116
 [8]: http://junit.org/junit4/javadoc/4.12/org/junit/rules/RuleChain.html
 [9]: https://developer.android.com/reference/android/app/Instrumentation.html#runOnMainSync(java.lang.Runnable)
 [10]: https://developer.android.com/reference/android/support/test/rule/UiThreadTestRule.html#runOnUiThread(java.lang.Runnable)