helpers.source() was a hack to work around the lack of anonymous
:source. Its "create tempfile" behavior is not a required part of most
tests that use it.

Some tests still need the old "create tempfile" behavior either because
they test SID behavior, or because of missing nvim_exec features: #16071

In vim, scrolling a window might mess up the cmdline. To keep it simple,
cmdline was always cleared for any window scroll. In nvim, where safe scrolling
is implemented in the TUI layer, this problem doesn't exist.

Clearing the message on scrolling, when we not do it e.g when switching tabs
is a bit weird, as the former is a much smaller context change.

A vim patch introduced the possibility to avoid the cmdlline clear for
redraws caused by async events. This case will now trivially be covered,
as the redraw is always avoided.

vim-patch:8.0.0592: if a job writes to a buffer screen is not updated

":normal :" might be invoked in various ways, so its safest to always
allow recursive invocation of cmdline mode