patch 9.0.1627: no generic mechanism to test syntax plugins

Problem:    No generic mechanism to test syntax plugins.
Solution:   Add a syntax plugin test mechanism, using screendumps.  Add a
            simple test for "c".

runtime/syntax/testdir/README.txt

@@ -0,0 +1,97 @@
+Tests for syntax highlighting plugins
+=====================================
+
+Summary: Files in the "input" directory are edited by Vim with syntax
+highlighting enabled.  Screendumps are generated and compared with the
+expected screendumps in the "dumps" directory.  This will uncover any
+character attributes that differ.
+
+Without any further setup a screendump is made at the top of the file (using
+_00.dump) and another one at the end of the file (using _99.dump).  The dumps
+are normally 20 screen lines tall.
+
+When the screendumps are OK an empty "done/{name}" file is created.  This
+avoids running the test again until "make clean" is used.  Thus you can run
+"make test", see one test fail, try to fix the problem, then run "make test"
+again to only repeat the failing test.
+
+When a screendump differs it is stored in the "failed" directory.  This allows
+for comparing it with the expected screendump, using a command like:
+
+	let fname = '{name}_99.dump'
+	call term_dumpdiff('failed/' .. fname, 'dumps/' .. fname)
+
+
+Creating a syntax plugin test
+-----------------------------
+
+Create a source file in the language you want to test in the "input"
+directory.  Make sure to include some interesting constructs with complicated
+highlighting.
+
+Use the filetype name as the base and a file name extension matching the
+filetype.  Let's use Java as an example.  The file would then be
+"input/java.java".
+
+If there is no further setup required, you can now run the tests:
+	make test
+
+The first time this will fail with an error for a missing screendump.  The
+newly created screendumps will be "failed/java_00.dump",
+"failed/java_01.dump", etc.  You can inspect each with:
+
+	call term_dumpload('failed/java_00.dump')
+	call term_dumpload('failed/java_01.dump')
+	...
+	call term_dumpload('failed/java_99.dump')
+
+If they look OK, move them to the "dumps" directory:
+
+	:!mv failed/java_00.dump dumps
+	:!mv failed/java_01.dump dumps
+	...
+	:!mv failed/java_99.dump dumps
+
+If you now run the test again, it will succeed.
+
+
+Adjusting a syntax plugin test
+------------------------------
+
+If you make changes to the syntax plugin, you should add code to the input
+file to see the effect of these changes.  So that the effect of the changes
+are covered by the test.  You can follow these steps:
+
+1. Edit the syntax plugin somewhere in your personal setup.  Use a file
+   somewhere to try out the changes.
+2. Go to the directory where you have the Vim code checked out and replace the
+   syntax plugin.  Run the tests: "make test".  Usually the tests will still
+   pass, but if you fixed syntax highlighting that was already visible in the
+   input file, carefully check that the changes in the screendump are
+   intentional:
+	let fname = '{name}_99.dump'
+	call term_dumpdiff('failed/' .. fname, 'dumps/' .. fname)
+   Fix the syntax plugin until the result is good.
+2. Edit the input file for your language to add the items you have improved.
+   (TODO: how to add another screendump?).
+   Run the tests and you should get failures.  Like with the previous step,
+   carefully check that the new screendumps in the "failed" directory are
+   good.  Update the syntax plugin and the input file until the highlighting
+   is good and you can see the effect of the syntax plugin improvements.  Then
+   move the screendumps from the "failed" to the "dumps" directory.  Now "make
+   test" should succeed.
+3. Prepare a pull request with the modified files:
+	- syntax plugin:    syntax/{name}.vim
+	- test input file:  syntax/testdir/input/{name}.{ext}
+	- test dump files:  syntax/testdir/dumps/{name}_99.dump
+
+As an extra check you can temporarily put back the old syntax plugin and
+verify that the tests fail.  Then you know your changes are covered by the
+test.
+
+
+
+TODO: run test for one specific filetype
+
+TODO: testing with various option values
+TODO: test syncing by jumping around