patch 8.2.0705: indent tests don't run on CI for FreeBSD

Problem:    Indent tests don't run on CI for FreeBSD.
Solution:   Set modeline. (Ozaki Kiichi, closes #6048)
diff --git a/runtime/indent/testdir/runtest.vim b/runtime/indent/testdir/runtest.vim
index b234a1a..6bbd33c 100644
--- a/runtime/indent/testdir/runtest.vim
+++ b/runtime/indent/testdir/runtest.vim
@@ -10,6 +10,7 @@
 syn on
 set nowrapscan
 set report=9999
+set modeline
 
 au! SwapExists * call HandleSwapExists()
 func HandleSwapExists()