tests: print traces from tests

[hawkw]

In order to make debugging test failures easier, it's helpful to print
the traces emitted by the code being tested.

This branch enables traces in tests by creating a
tracing_subscriber::fmt subscriber and setting it as the default for
each test. We set with_test_writer(true) so that the subscriber's log
output can participate in libtest's output capturing; the logs from each
test are only printed if that test fails. I changed all the tests to
ensure that the #[tokio::test] macro creates a current-thread runtime
so that the default subscriber is set for all tasks in the test.