patch 8.1.0723: cannot easily run specific test when in src/testdir

Problem:    Cannot run specific test when in src/testdir the same was as in
            the src directory.
Solution:   Move build rule to src/testdir/Makefile.
diff --git a/src/testdir/Make_dos.mak b/src/testdir/Make_dos.mak
index ebd8dd1..2623fb1 100644
--- a/src/testdir/Make_dos.mak
+++ b/src/testdir/Make_dos.mak
@@ -66,7 +66,7 @@
 # Must run test1 first to create small.vim.
 # This rule must come after the one that copies the input files to dostmp to
 # allow for running an individual test.
-$(SCRIPTS) $(SCRIPTS_GUI) $(SCRIPTS_WIN32) $(NEW_TESTS): $(SCRIPTS_FIRST)
+$(SCRIPTS) $(SCRIPTS_GUI) $(SCRIPTS_WIN32) $(NEW_TESTS_RES): $(SCRIPTS_FIRST)
 
 report:
 	@echo ""
@@ -115,7 +115,7 @@
 newtests: newtestssilent
 	@if exist messages (findstr "SKIPPED FAILED" messages > nul) && type messages
 
-newtestssilent: $(NEW_TESTS)
+newtestssilent: $(NEW_TESTS_RES)
 
 .vim.res:
 	@echo $(VIMPROG) > vimcmd