[ML] Functional tests - stabilize forecast tests (elastic#192737)

## Summary

This PR stabilizes and re-enables the forecast tests in the ML
permission and results view test suites by waiting for global loading to
finish before checking the forecast `Run` button state.

### Details

I was able to reproduce the original failures by running the test suites
in my local browser with network speed throttled to `3G`: the forecast
modal opened and the `Run` button was disabled during global page
loading, so the enabled check failed. Introducing the global loading
wait fixed the tests for the throttled local runs.

Closes elastic#189565
Closes elastic#189537
Closes elastic#164381

x-pack/test/functional/apps/ml/anomaly_detection_result_views/forecasts.ts

@@ -42,8 +42,7 @@ export default function ({ getService }: FtrProviderContext) {
   describe('forecasts', function () {
     this.tags(['ml']);
 
-    // FLAKY: https://github.com/elastic/kibana/issues/164381
-    describe.skip('with single metric job', function () {
+    describe('with single metric job', function () {
       before(async () => {
         await esArchiver.loadIfNeeded('x-pack/test/functional/es_archives/ml/farequote');
         await ml.testResources.createDataViewIfNeeded('ft_farequote', '@timestamp');

x-pack/test/functional/apps/ml/permissions/full_ml_access.ts

@@ -176,9 +176,7 @@ export default function ({ getService }: FtrProviderContext) {
       });
 
       for (const testUser of testUsers) {
-        // FLAKY: https://github.com/elastic/kibana/issues/189565
-        // FLAKY: https://github.com/elastic/kibana/issues/189537
-        describe.skip(`(${testUser.user})`, function () {
+        describe(`(${testUser.user})`, function () {
           before(async () => {
             await ml.securityUI.loginAs(testUser.user);
           });

x-pack/test/functional/services/ml/forecast.ts

@@ -9,9 +9,10 @@ import expect from '@kbn/expect';
 
 import { FtrProviderContext } from '../../ftr_provider_context';
 
-export function MachineLearningForecastProvider({ getService }: FtrProviderContext) {
+export function MachineLearningForecastProvider({ getPageObject, getService }: FtrProviderContext) {
   const testSubjects = getService('testSubjects');
   const retry = getService('retry');
+  const headerPage = getPageObject('header');
 
   return {
     async assertForecastButtonExists() {
@@ -102,6 +103,7 @@ export function MachineLearningForecastProvider({ getService }: FtrProviderConte
     },
 
     async assertForecastModalRunButtonEnabled(expectedValue: boolean) {
+      await headerPage.waitUntilLoadingHasFinished();
       const isEnabled = await testSubjects.isEnabled('mlModalForecast > mlModalForecastButtonRun');
       expect(isEnabled).to.eql(
         expectedValue,