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_selectmode.vim b/src/testdir/test_selectmode.vim
index 705c4ce..285ccb4 100644
--- a/src/testdir/test_selectmode.vim
+++ b/src/testdir/test_selectmode.vim
@@ -4,7 +4,7 @@
 CheckNotGui
 CheckUnix
 
-source mouse.vim
+source util/mouse.vim
 
 " Test for select mode
 func Test_selectmode_basic()