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 a541748 to see if there was a reason this was done but did not see one.

Change checklist

• 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