patch 8.1.1888: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 474e638..5249d4e 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1537,12 +1537,12 @@
 
   let curbuf = bufnr('')
   call writefile(['some', 'text'], 'XotherName')
-  let buf = bufadd('XotherName')
+  let buf = 'XotherName'->bufadd()
   call assert_notequal(0, buf)
-  call assert_equal(1, bufexists('XotherName'))
+  eval 'XotherName'->bufexists()->assert_equal(1)
   call assert_equal(0, getbufvar(buf, '&buflisted'))
   call assert_equal(0, bufloaded(buf))
-  call bufload(buf)
+  eval buf->bufload()
   call assert_equal(1, bufloaded(buf))
   call assert_equal(['some', 'text'], getbufline(buf, 1, '$'))
   call assert_equal(curbuf, bufnr(''))