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_options.vim b/src/testdir/test_options.vim
index e4bf63a..0846466 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -340,12 +340,12 @@
   " Expand directories.
   call feedkeys(":set cdpath=./\<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_match(' ./samples/ ', @:)
-  call assert_notmatch(' ./summarize.vim ', @:)
+  call assert_notmatch(' ./util/summarize.vim ', @:)
   set cdpath&
 
   " Expand files and directories.
   call feedkeys(":set tags=./\<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match(' ./samples/.* ./summarize.vim', @:)
+  call assert_match(' ./samples/.* ./test10.in', @:)
 
   call feedkeys(":set tags=./\\\\ dif\<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"set tags=./\\ diff diffexpr diffopt', @:)