patch 8.1.1915: 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_popup.vim b/src/testdir/test_popup.vim
index 6c86d69..5b602cc 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -250,7 +250,7 @@
 
 func Test_noinsert_complete()
   func! s:complTest1() abort
-    call complete(1, ['source', 'soundfold'])
+    eval ['source', 'soundfold']->complete(1)
     return ''
   endfunc
 
@@ -403,7 +403,7 @@
     return 0
   else
     call complete_add('four1')
-    call complete_add('four2')
+    eval 'four2'->complete_add()
     call complete_check()
     call complete_add('four3')
     call complete_add('four4')
@@ -993,7 +993,7 @@
   if empty(g:compl_what)
     let g:compl_info = complete_info()
   else
-    let g:compl_info = complete_info(g:compl_what)
+    let g:compl_info = g:compl_what->complete_info()
   endif
   return ''
 endfunc