patch 8.2.1456: MS-Windows: test files are not deleted

Problem:    MS-Windows: test files are not deleted.
Solution:   use "del" instead of $(DEL).
diff --git a/src/testdir/Make_dos.mak b/src/testdir/Make_dos.mak
index 4ca8713..ba728df 100644
--- a/src/testdir/Make_dos.mak
+++ b/src/testdir/Make_dos.mak
@@ -53,16 +53,16 @@
 # Delete files that may interfere with running tests.  This includes some files
 # that may result from working on the tests, not only from running them.
 clean:
-	-if exist *.out $(DEL) *.out
-	-if exist *.failed $(DEL) *.failed
-	-if exist *.res $(DEL) *.res
+	-if exist *.out del *.out
+	-if exist *.failed del *.failed
+	-if exist *.res del *.res
 	-if exist $(DOSTMP) rd /s /q $(DOSTMP)
 	-if exist test.in del test.in
 	-if exist test.ok del test.ok
 	-if exist Xdir1 rd /s /q Xdir1
 	-if exist Xfind rd /s /q Xfind
 	-if exist XfakeHOME rd /s /q XfakeHOME
-	-if exist X* $(DEL) X*
+	-if exist X* del X*
 	-for /d %i in (X*) do @rd /s/q %i
 	-if exist viminfo del viminfo
 	-if exist test.log del test.log
diff --git a/src/version.c b/src/version.c
index e873a9c..cc3b9fd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1456,
+/**/
     1455,
 /**/
     1454,