WIP(tests): Hide 'select_cases' rule

It doesn't provide any more information to expose it.

grammar.js

@@ -123,7 +123,7 @@ module.exports = grammar({
       "(",
       choice($.select_value, $.soong_config_variable),
       ",",
-      $.select_cases,
+      $._select_cases,
       ")",
     ),
 
@@ -151,7 +151,7 @@ module.exports = grammar({
       ")",
     ),
 
-    select_cases: ($) => seq(
+    _select_cases: ($) => seq(
       "{",
       optional(trailingCommaSeparated($.select_case)),
       // default *must* be the last one, enforced at parse-time...