[Tests] update plugins_discovery test

Updating a test that discovers plugins to have the correct amount.
The original tests removed the creation of the mock related to x-pack.

Update the test to get the correct values while running tests.

Signed-off-by: Kawika Avilla <kavilla414@gmail.com>

src/core/server/plugins/discovery/plugins_discovery.test.ts

@@ -154,8 +154,8 @@ describe('plugins discovery system', () => {
     const plugins = await plugin$.pipe(toArray()).toPromise();
     const pluginNames = plugins.map((plugin) => plugin.name);
 
-    expect(pluginNames).toHaveLength(3);
-    expect(pluginNames).toEqual(expect.arrayContaining(['pluginA', 'pluginB', 'pluginC']));
+    expect(pluginNames).toHaveLength(2);
+    expect(pluginNames).toEqual(expect.arrayContaining(['pluginA', 'pluginB']));
   });
 
   it('return errors when the manifest is invalid or incompatible', async () => {