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>
diff --git a/Filelist b/Filelist
index 34b1a23..10cddd0 100644
--- a/Filelist
+++ b/Filelist
@@ -878,6 +878,7 @@
 		runtime/syntax/testdir/input/setup/*.* \
 		runtime/syntax/testdir/dumps/*.dump \
 		runtime/syntax/testdir/dumps/*.vim \
+		runtime/syntax/testdir/tools/* \
 		runtime/syntax/generator/Makefile \
 		runtime/syntax/generator/README.md \
 		runtime/syntax/generator/gen_syntax_vim.vim \