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_startup_utf8.vim

@@ -63,7 +63,7 @@ endfunc
 
 func Test_detect_ambiwidth()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   " Use the title termcap entries to output the escape sequence.