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/runtest.vim b/src/testdir/runtest.vim
index a56679c..7822da9 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -101,13 +101,13 @@
 set shellslash
 
 " Common with all tests on all systems.
-source setup.vim
+source util/setup.vim
 
 " Needed for RunningWithValgrind().
-source shared.vim
+source util/shared.vim
 
 " Needed for the various Check commands
-source check.vim
+source util/check.vim
 
 " For consistency run all tests with 'nocompatible' set.
 " This also enables use of line continuation.