Minor issues fixed while trying to write browser tests in plugins (#11214)

* [contributing.md] fix command for testing plugins

* [ui/chrome] add navLinkExists() method

* [mocha] expose describe.skip

CONTRIBUTING.md

@@ -245,10 +245,11 @@ npm run test:dev
 
 This should work super if you're using the [Kibana plugin generator](https://github.com/elastic/generator-kibana-plugin). If you're not using the generator, well, you're on your own. We suggest you look at how the generator works.
 
-To run the tests for just your particular plugin, assuming you plugin lives outside of the `plugins directory`, use the following command.
+To run the tests for just your particular plugin run the following command from your plugin:
 
 ```bash
-npm run test:dev -- --kbnServer.testsBundle.pluginId=some_special_plugin --kbnServer.plugin-path=../some_special_plugin
+npm run test:server
+npm run test:browser -- --dev # remove the --dev flag to run them once and close
 ```
 
 ### Cross-browser Compatibility

src/ui/public/chrome/api/nav.js

@@ -6,6 +6,10 @@ export default function (chrome, internals) {
     return internals.nav;
   };
 
+  chrome.navLinkExists = (id) => {
+    return !!internals.nav.find(link => link.id === id);
+  };
+
   chrome.getNavLinkById = (id) => {
     const navLink = internals.nav.find(link => link.id === id);
     if (!navLink) {

src/ui/public/test_harness/test_sharding/setup_top_level_describe_filter.js

@@ -93,6 +93,7 @@ export function setupTopLevelDescribeFilter(test) {
 
   // to allow describe.only calls. we dont need interceptor as it will call describe internally
   describeInterceptor.only = originalDescribe.only;
+  describeInterceptor.skip = originalDescribe.skip;
 
   // ensure that window.describe isn't messed with by other code
   Object.defineProperty(window, 'describe', {