patch 8.2.1675: MinGW: testdir makefile deletes non-existing file

Problem:    MinGW: testdir makefile deletes non-existing file.
Solution:   Use another way to delete the output file if it already exists.
            (Michael Soyka)
diff --git a/src/testdir/Make_ming.mak b/src/testdir/Make_ming.mak
index f9dcfc8..5e17963 100644
--- a/src/testdir/Make_ming.mak
+++ b/src/testdir/Make_ming.mak
@@ -95,7 +95,7 @@
 # Copy the input files to dostmp, changing the fileformat to dos.
 $(DOSTMP)/%.in : %.in
 	if not exist $(DOSTMP)\nul mkdir $(DOSTMP)
-	if not exist $@ $(DEL) $@
+	if exist $(DOSTMP)\$< $(DEL) $(DOSTMP)\$<
 	$(VIMPROG) -u dos.vim $(NO_INITS) "+set ff=dos|f $@|wq" $<
 
 # For each input file dostmp/test99.in run the tests.