patch 8.1.1909: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make a few more functions usable as a method.
diff --git a/src/testdir/test_bufline.vim b/src/testdir/test_bufline.vim
index a6b4694..6d62e51 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -152,11 +152,11 @@
 
   let lines =<< trim END
     new foo
-    let winnr=bufwinnr('foo')
-    let buf=bufnr('foo')
+    let winnr = 'foo'->bufwinnr()
+    let buf = bufnr('foo')
     wincmd p
     call appendbufline(buf, '$', range(1,200))
-    exe winnr. 'wincmd w'
+    exe winnr .. 'wincmd w'
     norm! G
     wincmd p
     call deletebufline(buf, 1, '$')