tests: Add then for Authentication to Dex

Add single `then` to handle Dex authentication success or fail, so that
we can add a single `then` to check Dex authentication result

tests/post/steps/test_authentication.py

@@ -230,6 +230,42 @@ def successful_login(host, context, status_code):
     assert auth_response.headers.get('location') is not None
 
 
+@then(parsers.parse(
+    "we are able to login to Dex as '{username}' using password '{password}'"))
+def dex_login_success(
+    host,
+    control_plane_ip,
+    username,
+    password,
+    context,
+    request_retry_session
+):
+    dex_login(
+        host, control_plane_ip,
+        username, password,
+        context, request_retry_session
+    )
+    successful_login(host, context, '303')
+
+
+@then(parsers.parse(
+    "we are not able to login to Dex as '{username}' using password "
+    "'{password}'"))
+def dex_login_fail(
+    host,
+    control_plane_ip,
+    username,
+    password,
+    context,
+    request_retry_session
+):
+    dex_login(
+        host, control_plane_ip,
+        username, password,
+        context, request_retry_session
+    )
+    failed_login(host, context)
+
 #  }}}