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_amiga.mak b/src/testdir/Make_amiga.mak
index 6cb3ed8..b71b220 100644
--- a/src/testdir/Make_amiga.mak
+++ b/src/testdir/Make_amiga.mak
@@ -19,7 +19,7 @@
 SCRIPTS = $(SCRIPTS_ALL) $(SCRIPTS_MORE4)
 
 # Must run test1 first to create small.vim.
-$(SCRIPTS) $(SCRIPTS_GUI) $(NEW_TESTS): $(SCRIPTS_FIRST)
+$(SCRIPTS) $(SCRIPTS_GUI) $(NEW_TESTS_RES): $(SCRIPTS_FIRST)
 
 .SUFFIXES: .in .out