Added test for disabled plugin does not exist warning

Author: drpayyne

testData/inspections/xml/PluginDeclarationInspection/disabledNonExistingPlugin/di.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<!--
+/*
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<config>
+    <type name="Magento\CatalogSearch\Helper\Data">
+        <plugin name=<warning descr="This plugin does not exist to be disabled.">"plugin_which_does_not_exist"</warning> disabled="true" />
+    </type>
+</config>

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

@@ -35,6 +35,14 @@ public void testPluginNameDuplicationWarningWontShow() {
         myFixture.testHighlighting(true, false, false);
     }
 
+    /**
+     * Tests warning for disabling of non-existing plugin
+     */
+    public void testDisabledNonExistingPlugin() {
+        myFixture.configureByFile(getFixturePath(ModuleDiXml.FILE_NAME));
+        myFixture.testHighlighting(true, false, false);
+    }
+
     /**
      * Tests whenever the duplication warning shows when the plugin name already
      * defined in the same di.xml file