patch 8.1.0545: when executing indent tests user preferences interfere

Problem:    When executing indent tests user preferences interfere.
Solution:   Add "--clean".
diff --git a/runtime/indent/Makefile b/runtime/indent/Makefile
index fa81db0..3791f33 100644
--- a/runtime/indent/Makefile
+++ b/runtime/indent/Makefile
@@ -6,8 +6,8 @@
 # If a test succeeds a testdir/*.out file will be written.
 # If a test fails a testdir/*.fail file will be written.
 test:
-	$(VIM) --not-a-term -u testdir/runtest.vim
+	$(VIM) --clean --not-a-term -u testdir/runtest.vim
 
 
 clean:
-	$(VIM) --not-a-term -u testdir/cleantest.vim
+	$(VIM) --clean --not-a-term -u testdir/cleantest.vim