remove bucket env var req. & stop deleting all buckets

The system tests for error reporting were still not safe to run
concurrently. The issue is that at the end we were deleting all events
(there is no API to delete events selectively, AFAICT). This meant that
concurrent executions of the system tests against the same project could
clobber anothers logged errors.

This is consistent with the observed timeouts looking for events to
show up.

Fixes: #267

The system tests for error reporting were still not safe to run
concurrently. The issue is that at the end we were deleting all events
(there is no API to delete events selectively, AFAICT). This meant that
concurrent executions of the system tests against the same project could
clobber anothers logged errors.

This is consistent with the observed timeouts looking for events to
show up.

Fixes: #267

* The timeout used for waiting for the API to report an error event has been increased.
* The system tests only poll for error events for a specific service and version.
* The page size has been increased when requesting reported errors.
* Increase polling timeout to avoid using the API too much to hit quota exceeded errors.
* Error items are not deleted after the test.  This is needed to avoid conflicting with concurrent runs of the system test because it is not possible to delete error items only associated with a particular test.