pkg/server/api/agent/v1.TestAttestAgent has a data race in CI/CD

[zmt]

• Version: c5bc6baee2ead4022e73bdfd8ffe722ca3cbb3ea (HEAD of main)
• Platform: ubuntu-18.04 ci/cd environment
• Subsystem: server

Race detector detected a race in CI/CD: https://gist.github.com/zmt/97c57a223bec16e79e6cb85e37d62e93.

It is difficult to reproduce. It happens very infrequently (1 in 100 or so) and there's a limit of -count=25 because more will run out of pregenerated keys. It doesn't appear to be specific to a subtest. Most recent repro was a different one from the failures in CI/CD: https://gist.github.com/zmt/625ddfd1afb32688388c0a7ba882d898. In every case, it appears to be a race between spiretest.AssertLogs and logrus.