Modify test suite to dump junit XML

[dandavison]

As in temporalio/sdk-python#617 and temporalio/cli#650, I propose to modify the test suite to output junit XML. The motivation for this is that, given appropriate tools, it will make it easier for us to study patterns of flakiness, test timings etc, across matrix builds within a run, and across runs. Producing the files now will allow us to test tools for doing this.