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_clientserver.vim b/src/testdir/test_clientserver.vim
index 3d69eed..6fe0f69 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -8,7 +8,7 @@
 
 CheckFeature clientserver
 
-source shared.vim
+source util/shared.vim
 
 func Check_X11_Connection()
   if has('x11')