patch 8.1.0185: running tests writes lua.vim even though it is not used

Problem:    Running tests writes lua.vim even though it is not used.
Solution:   Stop writing lua.vim.
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 459b868..57bcb3a 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -51,7 +51,7 @@
 $(SCRIPTS) $(SCRIPTS_GUI) $(NEW_TESTS): $(SCRIPTS_FIRST)
 
 RM_ON_RUN = test.out X* viminfo
-RM_ON_START = tiny.vim small.vim mbyte.vim mzscheme.vim lua.vim test.ok benchmark.out
+RM_ON_START = tiny.vim small.vim mbyte.vim mzscheme.vim test.ok benchmark.out
 RUN_VIM = VIMRUNTIME=$(SCRIPTSOURCE); export VIMRUNTIME; $(VALGRIND) $(VIMPROG) -f $(GUI_FLAG) -u unix.vim $(NO_INITS) -s dotest.in
 
 clean: