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>

runtime/syntax/testdir/input/selftestdir/dots_18

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