Refactored Admin API test agent to use async/await

no issue

- Improves readability of what's going on in the code.

test/utils/admin-api-test-agent.js

@@ -20,13 +20,13 @@ class AdminAPITestAgent extends TestAgent {
     }
 
     async loginAs(email, password) {
-        await this.post('/session/')
+        const res = await this.post('/session/')
             .body({
                 grant_type: 'password',
                 username: email,
                 password: password
-            })
-            .then(function then(res) {
+            });
+
         if (res.statusCode === 302) {
             // This can happen if you already have an instance running e.g. if you've been using Ghost CLI recently
             throw new errors.IncorrectUsageError({
@@ -39,7 +39,6 @@ class AdminAPITestAgent extends TestAgent {
         }
 
         return res.headers['set-cookie'];
-            });
     }
 
     async loginAsOwner() {