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_ming.mak b/src/testdir/Make_ming.mak
index 7b38be5..2fa3e78 100644
--- a/src/testdir/Make_ming.mak
+++ b/src/testdir/Make_ming.mak
@@ -38,7 +38,7 @@
 SCRIPTS_BENCH = bench_re_freeze.out
 
 # Must run test1 first to create small.vim.
-$(SCRIPTS) $(SCRIPTS_GUI) $(SCRIPTS_WIN32) $(NEW_TESTS): $(SCRIPTS_FIRST)
+$(SCRIPTS) $(SCRIPTS_GUI) $(SCRIPTS_WIN32) $(NEW_TESTS_RES): $(SCRIPTS_FIRST)
 
 .SUFFIXES: .in .out .res .vim
 
@@ -110,7 +110,7 @@
 # to write and a lot easier to read and debug.
 # Limitation: Only works with the +eval feature.
 
-newtests: $(NEW_TESTS)
+newtests: $(NEW_TESTS_RES)
 
 .vim.res:
 	@echo $(VIMPROG) > vimcmd