Trust unit tests should target additional threat scenarios

This adds unit tests for two threat scenarios around the trust functionality
that are not currently tested.

The first scenario is related to deletion of a grant that has been previously
delegated via a trust. We need to ensure that executing a trust for a role that
the trustor no longer has is rejected.

The second scenario is related to an attempt to use a trust token with
impersonation to execute another trust as the impersonated user. We need to
ensure that a trust token can't be used to execute another trust.

SecurityImpact
Closes-Bug: #1347909
Change-Id: Ie1a0c286ff7e513cd964d4a93855230c78b98c6c
Merge "Trust unit tests should target additional threat scenarios"