logging: fix TestLogsAndDelete

[jeanbza]

Too many runs causes us to run out of quota. We've tried and failed to dramatically increase quota (logging team gives a ridiculously low quota of, like, 20 per hour to Delete, and won't budge on it).

We should probably use a project pool.

https://sponge.corp.google.com/target?id=35806e80-802d-4ab2-8c8a-006e70d5b9aa&target=cloud-go-libraries/google-cloud-go/continuous/go111&searchFor=&show=ALL&sortBy=STATUS

=== RUN   TestLogsAndDelete
--- FAIL: TestLogsAndDelete (0.08s)
    logging_test.go:497: deleting "GO-LOGGING-CLIENT/TEST-LOG-20191021-67194905680597-0001": rpc error: code = ResourceExhausted desc = Quota exceeded for quota metric 'logging.googleapis.com/control_requests' and limit 'ControlRequestsPerMinutePerProject' of service 'logging.googleapis.com' for consumer 'project_number:36639933145'.

[freelerobot]

Taking a look at this now

Ruled out pool project: tbp@: Project pools would be a bit more work to maintain. So, we've held off in favor of tests not assuming they have full control of the project. That's a bit easier for client library code than samples. Will look at alternative approach where we just delete once instead of all logs.

Ruled out making Logs return less logs: I explored extending Logs() so we can add a filter to it, and thus get less logs to be deleted. But this is not possible atm. #3108