Test Request: Add an E2E test for failovers with`vtbackup`

[EtienneBerube]

Overview of the Issue

This PR caught an issue with vtbackup's ability to progress if a failover happens during the catchup phase of a backup.

Ideally, this would have been caught early with an E2E test.

Solution

The vtbackup E2E test suite should have a test to simulate the loss of a primary, leading to vtbackup changing it's replication source.