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/vim9_tuples.vim

@@ -0,0 +1,25 @@
+vim9script
+# Vim9 tuples
+
+
+# https://github.com/vim/vim/pull/16935#issuecomment-2738310273
+
+function LegacyMakeEntry(key, value) abort
+    return (a:key, a:value)
+endfunction
+
+def MakeEntry(key: string, value: any): tuple<string, any>
+    return (key, value)
+enddef
+
+function LegacyMakeLazyList(e1, e2) abort
+    return ({e1_, e2_ -> {-> [e1_, e2_]}}(a:e1, a:e2))
+endfunction
+
+def MakeLazyList(e1: any, e2: any): func(): list<any>
+    return (((e1_: any, e2_: any) => () => [e1_, e2_])(e1, e2))
+enddef
+
+echo MakeEntry('key', 'value') == list2tuple(MakeLazyList('key', 'value')())
+echo LegacyMakeEntry('key', 'value') == list2tuple(LegacyMakeLazyList('key', 'value')())
+