Flakey test TestAgentRegistersSessionsWithSessionTracker

[ijc]

In testing of #2221/#2224 I saw this twice, once with master (out of 20 iterations, labelled before in the attached logs) and once more with the fix from #2224 + my grpc update applied (again out of 20 iterations, labelled fix in the attached logs). So this is a quite low rate of flakiness.

In both cases the fail was:

--- FAIL: TestAgentRegistersSessionsWithSessionTracker (0.15s)
        <autogenerated>:12: ^M                        ^M        Error Trace:    agent_test.go:447
                ^M      Error:          Not equal: 0 (expected)
                                != 1 (actual)

Logs:

• test-before.18.log from master
• test-fix.18.log with [ca] GetRemoteSignedCertificate downloads cert in Issuance or Rotate state #2224 and grpc update.