Added test for disabled observer inspection warning

Author: drpayyne

testData/inspections/xml/ObserverDeclarationInspection/disablingNonExistingObserver/events.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0"?>
+<!--
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<config>
+    <event name="test_event_in_test_class">
+        <observer name=<warning descr="This observer does not exist to be disabled. For more details, see Inspection Description.">"test_non_existing_observer"</warning>
+                  instance="Magento\Catalog\Observer\TestObserver"
+                  disabled="true"/>
+    </event>
+</config>

tests/com/magento/idea/magento2plugin/inspections/xml/ObserverDeclarationInspectionTest.java

@@ -31,4 +31,12 @@ public void testObserverNameUsedInDifferentFile() {
         myFixture.configureByFile(getFixturePath(ModuleEventsXml.FILE_NAME));
         myFixture.testHighlighting(true, false, false);
     }
+
+    /**
+     * Tests warning for disabling of non-existing observer.
+     */
+    public void testDisablingNonExistingObserver() {
+        myFixture.configureByFile(getFixturePath(ModuleEventsXml.FILE_NAME));
+        myFixture.testHighlighting(true, false, false);
+    }
 }