Tests: adapt stream_js_preload_object.t to nginx changes.

Tests: adapt stream_js_preload_object.t to nginx changes.

Make the test more robust against changes in nginx, specifically
cf890df37bb6 (Stream: socket peek in preread phase).

The filter callbacks may be called multiple times by nginx and the exact
number is not specified. The new test avoids relying on the exact number
of calls from nginx.