Apparently setting focus doesn't work until a mouse click is simulated
to bring up the main test window to the front. This happened implicitly
when running the full test suite, due to e.g. doing it in wxButton unit
tests, which come before wxComboBox::ProcessEnter test, but not when
running just this test (or wxTextCtrl::ProcessEnter) on its own.
Make the test self-contained by simulating a mouse click before
simulating pressing "Enter".