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_vimscript.vim b/src/testdir/test_vimscript.vim
index e1000fb..af78d1c 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -2,7 +2,7 @@
 " Most of this was formerly in test49.vim (developed by Servatius Brandt
 " <Servatius.Brandt@fujitsu-siemens.com>)
 
-source script_util.vim
+source util/script_util.vim
 
 "-------------------------------------------------------------------------------
 " Test environment							    {{{1
@@ -23,7 +23,7 @@
 func RunInNewVim(test, verify)
   let init =<< trim END
     set cpo-=C            " support line-continuation in sourced script
-    source script_util.vim
+    source util/script_util.vim
     XpathINIT
     XloopINIT
   END