vim-patch:8.1.1634: terminal test fails when term_getansicolors() is missing

Problem:    Terminal test fails when term_getansicolors() is missing.
            Diff test fails without +rightleft.  (Dominique Pelle)
Solution:   Check if term_getansicolors() is supported. (closes vim/vim#4597)
981d9dc8f3

src/nvim/testdir/test_diffmode.vim

@@ -752,6 +752,9 @@ func Test_diff_of_diff()
   if !CanRunVimInTerminal()
     return
   endif
+  if !has("rightleft")
+    throw 'Skipped: rightleft not supported'
+  endif
 
   call writefile([
 	\ 'call setline(1, ["aa","bb","cc","@@ -3,2 +5,7 @@","dd","ee","ff"])',