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/Makefile b/src/Makefile
index 7e2a843..e00c573 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1565,9 +1565,12 @@
 DEST_MAN_RU = $(DEST_MAN_TOP)/ru.KOI8-R$(MAN1DIR)
 DEST_MAN_RU_U = $(DEST_MAN_TOP)/ru.UTF-8$(MAN1DIR)
 
-# The list of tests is common to all systems.
+# stuff common to all systems
 include Make_all.mak
 
+# get the list of tests
+include testdir/Make_all.mak
+
 #	     BASIC_SRC: files that are always used
 #	       GUI_SRC: extra GUI files for current configuration
 #	   ALL_GUI_SRC: all GUI files for Unix
@@ -2184,11 +2187,7 @@
 # Run individual NEW style test.
 # These do not depend on the executable, compile it when needed.
 $(NEW_TESTS):
-	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile $@.res VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
-	@if test -f testdir/test.log; then \
-		cat testdir/test.log; \
-	fi
-	cat testdir/messages
+	cd testdir; $(MAKE) $@ VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
 
 newtests:
 	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile newtestssilent VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)