frame aggregation test coverage

In the default case it makes sense to be as transparent as possible, forwarding all frames as-is. When `aggregate_fragments` is turned on, however, we are no longer a completely neutral intermediary, so we need to do a little more work to detect protocol errors and incorrect behavior.

[This](https://github.com/crossbario/autobahn-testsuite) is a test suite for WebSockets with many helpful test cases around fragmentation. It would be good to implement this in CI or translate the fragmentation test cases into `Test::Nginx::Socket`.

Start by reviewing the repository's existing CI configuration and Test::Nginx::Socket tests, then compare their coverage with the fragmentation cases in the linked autobahn-testsuite. Done means the relevant frame-aggregation protocol errors and incorrect behaviors are covered by automated tests, either by integrating the suite into CI or translating its cases.

Written by the indexing model from the issue text.