[3.15] gh-69134: Wait until mapped in SimpleDialog keyboard tests (GH-152690) (GH...

[3.15] gh-69134: Wait until mapped in SimpleDialog keyboard tests (GH-152690) (GH-153104)

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 these tests,
as GH-152599 did for the other keyboard tests.

(cherry picked from commit 7d619c2fea131722d0bfdad150c081c44cc0abd7)

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