patch 8.2.1498: on slow systems tests can be flaky

Problem:    On slow systems tests can be flaky.
Solution:   Use TermWait() instead of term-wait(). (Yegappan Lakshmanan,
            closes #6756)
diff --git a/src/testdir/test_digraph.vim b/src/testdir/test_digraph.vim
index 7937321..22c1330 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -504,13 +504,13 @@
   CheckRunVimInTerminal
   let buf = RunVimInTerminal('', {'rows': 6})
   call term_sendkeys(buf, "i\<C-K>")
-  call term_wait(buf)
+  call TermWait(buf)
   call assert_equal('?', term_getline(buf, 1))
   call term_sendkeys(buf, "1")
-  call term_wait(buf)
+  call TermWait(buf)
   call assert_equal('1', term_getline(buf, 1))
   call term_sendkeys(buf, "2")
-  call term_wait(buf)
+  call TermWait(buf)
   call assert_equal('½', term_getline(buf, 1))
   call StopVimInTerminal(buf)
 endfunc