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_diffmode.vim

@@ -723,7 +723,7 @@ endfunc
 
 func Test_diff_with_cursorline()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   call writefile([
@@ -750,7 +750,7 @@ endfunc
 
 func Test_diff_of_diff()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
   if !has("rightleft")
     throw 'Skipped: rightleft not supported'