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>

src/Makefile

@@ -3022,6 +3022,7 @@ shadow:	runtime pixmaps
 				 ../../testdir/ru_RU \
 				 ../../testdir/sautest \
 				 ../../testdir/samples \
+				 ../../testdir/util \
 				 ../../testdir/dumps \
 				 ../../testdir/*.ok \
 				 ../../testdir/testluaplugin \