gh-69134: Wait until mapped in SimpleDialog keyboard tests (GH-152690)

gh-69134: Wait until mapped in SimpleDialog keyboard tests (GH-152690)

The SimpleDialog keyboard tests generate key events after focus_force(),
which on Windows are dropped until the toplevel is mapped, so they could
fail intermittently (seen on the Windows10 buildbot as
test_return_no_default).  Wait until the window is mapped in each of
these tests, as GH-152599 did for the other keyboard tests.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>