patch 8.1.1213: "make clean" in top dir does not cleanup indent test output

Problem:    "make clean" in top dir does not cleanup indent test output.
Solution:   Clean the indent test output.  Do not rely on the vim executable
            for that. (closes #4307)
diff --git a/runtime/indent/Makefile b/runtime/indent/Makefile
index 1917e46..d192605 100644
--- a/runtime/indent/Makefile
+++ b/runtime/indent/Makefile
@@ -11,4 +11,4 @@
 
 
 clean:
-	VIMRUNTIME=$(VIMRUNTIME) $(VIM) --clean --not-a-term -u testdir/cleantest.vim
+	rm -f testdir/*.fail testdir/*.out