Less SPOFs: pyjunitxml, testscenarios

I’ve made the Testtools committers team own both the project and the trunk branch for both pyjunitxml and testscenarios. This removes me as a SPOF if anything needs doing in those projects – any Testtools committer can now do it. (Including code review and landing). If you are a testtools committer and need PyPI release rights, ping me and I’ll add you. (I wish PyPI had group management).