Assert the specific type of event, not just the existence of any event.

Also remove a bunch of custom uses of EventCollector, and use the (already
existing) EventCollectionApparatus instead.

--
MOS_MIGRATED_REVID=106047665
diff --git a/src/test/java/com/google/devtools/build/lib/packages/PackageGroupTest.java b/src/test/java/com/google/devtools/build/lib/packages/PackageGroupTest.java
index 9d501fc..5d2ab8d 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/PackageGroupTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/PackageGroupTest.java
@@ -51,7 +51,7 @@
 
     events.setFailFast(false);
     getPackage("strawberry");
-    events.assertContainsEvent("package group has invalid name");
+    events.assertContainsError("package group has invalid name");
   }
 
   @Test
@@ -81,7 +81,7 @@
 
     events.setFailFast(false);
     getPackageGroup("fruits", "apple");
-    events.assertContainsEvent("invalid package label: vegetables");
+    events.assertContainsError("invalid package label: vegetables");
   }
 
   @Test
@@ -96,7 +96,7 @@
 
     events.setFailFast(false);
     getPackageGroup("fruits", "apple");
-    events.assertContainsEvent("invalid package label: //vegetables:carrot");
+    events.assertContainsError("invalid package label: //vegetables:carrot");
   }
 
   @Test
@@ -109,7 +109,7 @@
 
     events.setFailFast(false);
     getPackageGroup("fruits", "apple");
-    events.assertContainsEvent("invalid package label: :carrot");
+    events.assertContainsError("invalid package label: :carrot");
   }
 
   @Test