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_number.vim b/src/testdir/test_number.vim
index 10b6988..bac01d1 100644
--- a/src/testdir/test_number.vim
+++ b/src/testdir/test_number.vim
@@ -1,6 +1,6 @@
 " Test for 'number' and 'relativenumber'
 
-source screendump.vim
+source util/screendump.vim
 
 func s:screen_lines(start, end) abort
   return ScreenLines([a:start, a:end], 8)