test: fix firefox stress test

[dozyio]

Description

Replaced generic drain-event waiting with the stream-native backpressure API in packages/interface-compliance-tests/src/stream-muxer/stress-test.ts:

• Changed await pEvent(outboundStream, 'drain', { rejectionEvents: ['close'] }) to await outboundStream.onDrain().
• This preserves behaviour (wait until writable; reject if stream closes/resets) while avoiding high per-iteration listener/promise overhead in the stress loop.
• Wrapped this.processSendQueue in queueMicrotask to avoid blowing up the stack.

Observed impact:

• Firefox: ~27–30s down to 3-4s.
• Chromium: 2-2.5s down to 1.5-2s

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