Fix malformed engine entry in build-ts-extension test fixture

The engines list takes either a string (engine name) or an object with
a path. The fixture had 'test-engine.js' as a bare string, which would
register an engine named 'test-engine.js' rather than point at the
bundled module.

Author: gordonwoodhull

tests/smoke/build-ts-extension/_extensions/test-engine/_extension.yml

@@ -4,4 +4,4 @@ version: 1.0.0
 quarto-required: ">=1.6.0"
 contributes:
   engines:
-    - test-engine.js
+    - path: test-engine.js