fix(mock-connection): use muxer to close/abort streams

[tabcat]

Title

fix(mock-connection): use muxer to close/abort streams

Description

Uses this.muxer.close and this.muxer.abort in MockConnection's close and abort methods.

Notes & open questions

Fixes #2675

I looked at https://github.com/libp2p/js-libp2p/commit/a541748c32c5644aa8098c81e96e51940cb25eaa to see if there was a reason this was done but did not see one.

Change checklist

• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation if necessary (this includes comments as well)
• [ ] I have added tests that prove my fix is effective or that my feature works