runtime(syntax-tests): Apply stronger synchronisation between buffers

runtime(syntax-tests): Apply stronger synchronisation between buffers

The current lightweight synchronisation with ":redraw" needs further
reinforcement in the light of v9.1.1110.  And, with v9.1.0820, make
another synchronisation point _before_ the first (or only) screenful is
dumped.

Also add a script to regenerate all screendumps.

closes: #16632

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>