Allow for env variable to set the username for E2E tests (#3345)

e2e/modules/environment.js

@@ -267,7 +267,8 @@ module.exports = {
         await window.waitForSelector('#input_password-input');
         await window.waitForSelector('#saveSetting');
 
-        let username;
+        let username = process.env.MM_TEST_USERNAME;
+        if (!username) {
             switch (process.platform) {
             case 'darwin':
                 username = 'success+sysadmin+macos@simulator.amazonses.com';
@@ -281,6 +282,7 @@ module.exports = {
             default:
                 throw new Error('Unsupported platform');
             }
+        }
 
         await window.type('#input_loginId', username);
         await window.type('#input_password-input', process.env.MM_TEST_PASSWORD);