patch 8.0.1553: cannot see what digraph is used to insert a character

Problem:    Cannot see what digraph is used to insert a character.
Solution:   Show the digraph with the "ga" command. (Christian Brabandt)
diff --git a/src/testdir/shared.vim b/src/testdir/shared.vim
index ef85514..5fef6bd 100644
--- a/src/testdir/shared.vim
+++ b/src/testdir/shared.vim
@@ -259,3 +259,14 @@
 func CanRunGui()
   return has('gui') && ($DISPLAY != "" || has('gui_running'))
 endfunc
+
+" Get line "lnum" as displayed on the screen.
+" Trailing white space is trimmed.
+func! Screenline(lnum)
+  let chars = []
+  for c in range(1, winwidth(0))
+    call add(chars, nr2char(screenchar(a:lnum, c)))
+  endfor
+  let line = join(chars, '')
+  return matchstr(line, '^.\{-}\ze\s*$')
+endfunc