patch 9.1.0763: tests: cannot run single syntax tests

Problem:  tests: cannot run single syntax tests
Solution: Support running a subset of syntax tests
          (Aliaksei Budavei)

Two methods of assembling a subset of test filenames for
selection are provided:

* Filename and filetype Make targets will be generated, and
  multiple such targets can be passed before the mandated
  trailing "test" target, e.g. "make html markdown test".

* Filenames and their parts can be specified as a regular
  expression that is assigned to a "VIM_SYNTAX_TEST_FILTER"
  environment variable, and used with the test Make target,
  e.g. "VIM_SYNTAX_TEST_FILTER=html\\\|markdown make test".
  (This variable will be ignored and the whole suite will be
  run when Make is GNU Make and a parent Makefile is used.)

Methods can be used alone or together, with the Make targets
having the higher precedence. Neither method will influence
the order of test execution.

closes: #15670

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

runtime/syntax/testdir/input/selftestdir/README.txt

@@ -6,5 +6,5 @@ This is mainly used for debugging and testing the syntax test suite.
 
 Please test any changes as follows:
 	cd runtime/syntax/
-	VIM_SYNTAX_SELF_TESTING=1 make clean test
+	make clean self-testing test