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_syntax.vim

@@ -496,8 +496,8 @@ endfunc
 
 " Check highlighting for a small piece of C code with a screen dump.
 func Test_syntax_c()
-  if !has('terminal')
-    return
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
   endif
   call writefile([
 	\ '/* comment line at the top */',