test(modal): add sanity check for flaky test in CI

[TomJGooding]

Note that the test will even fail locally with this change instead:

-        footer_key_clicked = await pilot.mouse_down("FooterKey")
+        footer_key_clicked = await pilot.click("FooterKey")
         assert footer_key_clicked is True  # Sanity check
         await pilot.pause()

>           assert footer_key_clicked is True  # Sanity check
E           assert False is True

Since the footer reacts to mouse down, when the click has finished on mouse up the screen is now the modal screen.

[TomJGooding]

And fails in CI with this change:

-        await pilot.pause(0.4)
+        await pilot.pause()

FAILED tests/test_modal.py::test_modal_pop_screen - textual.css.query.NoMatches: No nodes match 'FooterKey' on Screen(id='_default')

[TomJGooding]

Maybe there is a better fix, but this should hopefully unblock for the time being and at least the sanity check will help debug if the footer key is actually clicked.

[willmcgugan]

Going with the other branch. Although the fix isn't all that satisfactory, it avoids the arbitrary sleep which is often a red flag and addresses some genuine issues.