[Discover] Add permissions for context size test (elastic#109391)

* [Discover] add permissions for flaky context test

* [Discover] apply test_logstash_reader role

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

test/functional/apps/context/_size.ts

@@ -15,6 +15,7 @@ const TEST_STEP_SIZE = 2;
 
 export default function ({ getService, getPageObjects }: FtrProviderContext) {
   const kibanaServer = getService('kibanaServer');
+  const security = getService('security');
   const retry = getService('retry');
   const docTable = getService('docTable');
   const browser = getService('browser');
@@ -23,6 +24,7 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
 
   describe('context size', function contextSize() {
     before(async function () {
+      await security.testUser.setRoles(['kibana_admin', 'test_logstash_reader']);
       await kibanaServer.uiSettings.update({
         'context:defaultSize': `${TEST_DEFAULT_CONTEXT_SIZE}`,
         'context:step': `${TEST_STEP_SIZE}`,