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>
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 856a526..b5c9f63 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -3,8 +3,8 @@
 
 CheckFeature textprop
 
-source screendump.vim
-import './vim9.vim' as v9
+source util/screendump.vim
+import './util/vim9.vim' as v9
 
 func Test_proptype_global()
   call prop_type_add('comment', {'highlight': 'Directory', 'priority': 123, 'start_incl': 1, 'end_incl': 1})