patch 9.1.1525: tests: testdir/ is a bit messy

Problem:  tests: testdir is a bit messy
Solution: move test scripts into testdir/util/ directory

src/testdir/ has become a dumping ground mixing test cases with utility
functions. Let's fix this by moving all utility functions into the
testdir/util/ directory

Also a few related changes had to be done:
- Update Filelist
- update README.txt and mention the new directory layout
- fix shadowbuild by linking the util directory into the shadow dir

closes: #17677

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/runtest.vim

@@ -69,7 +69,8 @@ endif
 " Use the script for source code screendump testing.  It sources other scripts,
 " therefore we must "cd" there.
 cd ../../src/testdir
-source screendump.vim
+source util/screendump.vim
+source util/term_util.vim
 exe 'cd ' .. fnameescape(syntaxDir)
 
 " For these tests we need to be able to run terminal Vim with 256 colors.  On