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_netbeans.vim b/src/testdir/test_netbeans.vim
index cfb9736..d3d5e8b 100644
--- a/src/testdir/test_netbeans.vim
+++ b/src/testdir/test_netbeans.vim
@@ -876,7 +876,7 @@
   call delete("Xnetbeans")
   call writefile([], "Xnetbeans", 'D')
   let after =<< trim END
-    source shared.vim
+    source util/shared.vim
     set cpo&vim
 
     func ReadXnetbeans()