CBG-842 - Add TestBucket override for RestTester

[bbrks]

Allows tests to use multiple RestTester instances pointing to the same bucket (to simulate multi-node SG scenarios)

Example Usage:

tb := base.GetTestBucket(t)
defer tb.Close()

dbc := DbConfig{AutoImport: true, EnableXattrs: base.BoolPtr(true)}

rt1 := NewRestTester(t, &RestTesterConfig{TestBucket: tb, DatabaseConfig: &dbc})
defer rt1.Close()

rt2 := NewRestTester(t, &RestTesterConfig{TestBucket: tb, DatabaseConfig: &dbc})
defer rt2.Close()

[coveralls]

Coverage decreased (-0.008%) to 61.73% when pulling 6e16588 on CBG-842 into 21975e2 on master.