Tests: ust_constructor: convert left-over type hint to type comment

Use type comments to support older python3 interpreters that can't
handle type hints (such as 3.4).

Python 3.4 reports the following error:
  File "./ust/ust-constructor/test_ust_constructor.py", line 176
    client: lttngtest.Controller = lttngtest.LTTngClient(test_env, log=tap.diagnostic)
          ^
  SyntaxError: invalid syntax
  ERROR: ust/ust-constructor/test_ust_constructor.py - missing test plan
  ERROR: ust/ust-constructor/test_ust_constructor.py - exited with status 1

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ic0ead4b9bedfca56e8999bf4012e103801794655