test: remove dead throwingTag setup and duplicate throw stub

for.spec kept throwingTag registration after #713 removed its test. Reuse ThrowingTag in liquid.spec instead of IntendedRenderErrorTag.

Co-authored-by: Cursor <[email protected]>

test/stub/tags.ts

@@ -24,9 +24,3 @@ export class ThrowsOnParseTag extends Tag {
     return ''
   }
 }
-
-export class IntendedRenderErrorTag extends Tag {
-  render () {
-    throw new Error('intended render error')
-  }
-}