oldtest: more compact output with "clean" target (#10477)

Previously it would be displayed across multiple lines (with the escaped
newlines), while this makes it display in a single line.

src/nvim/testdir/Makefile

@@ -99,8 +99,7 @@ RM_ON_RUN   := test.out X* viminfo
 RM_ON_START := test.ok
 RUN_VIM     := $(TOOL) $(NVIM_PRG) -u unix.vim -U NONE -i viminfo --headless --noplugin -s dotest.in
 
-clean:
-	-rm -rf *.out          \
+CLEAN_FILES := *.out \
   *.failed          \
   *.res             \
   *.rej             \
@@ -116,6 +115,8 @@ clean:
   .gdbinit          \
   $(TMPDIR)         \
   del
+clean:
+	$(RM) -rf $(CLEAN_FILES)
 
 test1.out: .gdbinit test1.in
 	@echo "[OLDTEST-PREP] Running test1"