Add test: connected user can be retrieved (#247)

## What is the goal of this PR?

We've added a behaviour test scenario testing that the connected user is
retrievable. This checks that the client provides a way to get the
currently logged in user.

## What are the changes implemented in this PR?

* Added behaviour test scenario: `connected user is retrievable`.

connection/user.feature

@@ -36,6 +36,14 @@ Feature: Connection Users
     Given users delete: user
     Then users not contains: user
 
+  Scenario: connected user is retrievable
+    Given typedb starts
+    Given connection opens with authentication: admin, password
+    When users create: user, password
+    Then connection closes
+    Given connection opens with authentication: user, password
+    Then get connected user
+
   @ignore-typedb-client-python @ignore-typedb-client-nodejs
   Scenario: user passwords must comply with the minimum length
     Given typedb has configuration