e2e test MAX_TOTAL_RECORDS (against Spanner)

[pjenvey]

Spanner's mutation limit requires a lower number of MAX_TOTAL_RECORDS. The e2e tests should ensure max_total_records edge cases can commit successfully (to avoid regressions such as mozilla-services/syncstorage-rs#869

[pjenvey]

This is pretty straightforward to test but potentially a lengthy test to run every time the e2e tests are invoked.

[pjenvey]

Also logged here: https://mozilla-hub.atlassian.net/browse/SYNC-3525