v4 improve coverage (#25945)

* Improve code coverage for our Button plugin

* improve coverage for our Alert plugin

* test tooltip update method

* test update and dispose for dropdown

* increase code coverage requirements

js/tests/karma.conf.js

@@ -60,10 +60,10 @@ module.exports = (config) => {
       thresholds: {
         emitWarning: false,
         global: {
-          statements: 89,
-          lines: 89,
-          branches: 83,
-          functions: 84
+          statements: 90,
+          branches: 84,
+          functions: 87,
+          lines: 90
         }
       }
     }