Do not rely on timing but wait for Kakoune to redraw which is what
we actually need: Mouse clicks rely on the current display buffer
to resolve the buffer location, so we need to wait for a redraw
to happen with `ui_out`
The macOS CI manges to trigger this race. When it happens the
"c" inserted by the last command is not seen by the test runner.
Let's fix this by adding yet another sleep.
Recent changes for selection-undo added an assertion that triggers
when a mouse-drag overlaps with an insert mode, because both events
record selection history. However this is actually fine. The one
that finishes last concludes the selection edition, while the other
one will be a nop.
The test could be simpler (i.e. not require sleeps) but I figured it
doesn't hurt add this since we don't have any comparable tests.
