patch 8.2.0885: "make shadow" does not link new lua test dir

Problem:    "make shadow" does not link new lua test dir.
Solution:   Also link testdir/testluaplugin. (Elimar Riesebieter)
diff --git a/src/Makefile b/src/Makefile
index 520ef0b..c12481c 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2986,7 +2986,9 @@
 				 ../../testdir/samples \
 				 ../../testdir/dumps \
 				 ../../testdir/test83-tags? \
-				 ../../testdir/*.ok .
+				 ../../testdir/*.ok \
+				 ../../testdir/testluaplugin \
+				 .
 
 # After updating Vim new files may have been created, use this to refresh the
 # symbolic links in the shadow directory. This isn't guaranteed to catch all