runtime(syntax-tests): Introduce self tests for screen dumping

Write a batch of test files with made-up syntax to serve for
additional linewise checks to be manually performed whenever
the algorithm for screen dump file generation is modified.

Define a shell variable VIM_SYNTAX_SELF_TESTING to run these
tests:
	cd runtime/syntax/
	VIM_SYNTAX_SELF_TESTING=1 make clean test

related: #15150

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/Filelist b/Filelist
index 1652cf3..30c8208 100644
--- a/Filelist
+++ b/Filelist
@@ -836,6 +836,7 @@
 		runtime/syntax/testdir/runtest.vim \
 		runtime/syntax/testdir/ftplugin/*.* \
 		runtime/syntax/testdir/input/*.* \
+		runtime/syntax/testdir/input/selftestdir/* \
 		runtime/syntax/testdir/input/setup/*.* \
 		runtime/syntax/testdir/dumps/*.dump \
 		runtime/syntax/testdir/dumps/*.vim \