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_constructors.vim

@@ -0,0 +1,18 @@
+vim9script
+# Vim9 constructors
+
+
+class A
+  static var _instance: A
+  var str: string
+  def _new(str: string)
+    this.str = str
+  enddef
+  static def GetInstance(str: string): A
+    if _instance == null
+      _instance = A._new(str)
+    endif
+    return _instance
+  enddef
+endclass
+