vim-patch:8.1.1483: skipped tests are not properly listed

Problem:    Skipped tests are not properly listed.
Solution:   Throw a "Skipped" exception instead of using ":finish" or ":return".
5d30ff1964

Skips quite some (N/A) tests.

vim-patch:8.1.0503: missing change to diff test (included in cf1ffa916)

src/nvim/testdir/test_fold.vim

@@ -706,7 +706,7 @@ endfunc
 
 func Test_folds_with_rnu()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot make screendumps'
   endif
 
   call writefile([