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_line_continuation.vim

@@ -1,5 +1,6 @@
 " Vim line continuations with interspersed comments
 
+
 function Foo(
       "\ param a
       \ a,
@@ -47,3 +48,4 @@ let array = [
       "\ element 3
       \ 3
       \]
+