runtime(vim): Cleanup syntax tests

Improve formatting and naming consistency of the syntax tests.

closes: #17850

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_ex_def_nested.vim

@@ -2,6 +2,7 @@ vim9script
 # Vim9 :def command (nested)
 # VIM_TEST_SETUP hi link vim9This Todo
 
+
 class Test
     const name: string
 
@@ -19,3 +20,4 @@ class Test
 endclass
 
 echo Test.new()
+