UI Tester test time mismatch between wx and qt with same delay

See comment: https://github.com/enthought/traitsui/pull/1318#pullrequestreview-503942729

In summary, the same test was run on wx and qt with a UI Tester having `delay` parameter of 1000ms. On wx the test took 3.4 seconds to run and on qt 6.3. In the test a `KeySequence("NEW")` is performed so one would expect the test to run in ~3s as 1000ms for each key typed. Thus I suspect there may be some double delaying occurring in the test UI Tester code for qt, but I have not fully explored to confirm/identify the source of the discrepancy.