TestOutgoingTrailerMatcher is non-deterministic

[joshgarnett]

PR #4259 added comparing the headers in TestOutgoingTrailerMatcher. While working on another PR, I noticed that this would sometimes fail when the github hooks run the tests. This is caused by the TrailerMD being a map, which has a non-deterministic read order. A simple fix is to just sort the headers received before comparing.

I'll create a PR shortly.