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/Make_amiga.mak b/src/testdir/Make_amiga.mak
index 843ea1e..6ffc12d 100644
--- a/src/testdir/Make_amiga.mak
+++ b/src/testdir/Make_amiga.mak
@@ -21,7 +21,7 @@
 
 .in.out:
 	copy $*.ok test.ok
-	$(VIMPROG) -u amiga.vim -U NONE --noplugin --not-a-term -s dotest.in $*.in
+	$(VIMPROG) -u util/amiga.vim -U NONE --noplugin --not-a-term -s dotest.in $*.in
 	diff test.out $*.ok
 	rename test.out $*.out
 	-delete X#? ALL QUIET