patch 8.2.0935: flattening a list with existing code is slow

Problem:    Flattening a list with existing code is slow.
Solution:   Add flatten(). (Mopp, closes #3676)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 7402067..2a133ef 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -135,6 +135,7 @@
 	test_find_complete \
 	test_findfile \
 	test_fixeol \
+	test_flatten \
 	test_float_func \
 	test_fnameescape \
 	test_fnamemodify \
@@ -373,6 +374,7 @@
 	test_find_complete.res \
 	test_findfile.res \
 	test_fixeol.res \
+	test_flatten.res \
 	test_float_func.res \
 	test_fnameescape.res \
 	test_fold.res \