Improve coverage of storing credentials in parser.Stack

Currently the trusts path is not directly tested, so add coverage
and ensure the correct credentials are stored in each case.

Change-Id: I0aa999e01015046946f242a9b52b484522dc6d72
Add coverage for trusts parser.Stack delete path

Change-Id: Ic55030be389ac71ec999e08533fa9d5fc05b5bd1
Merge "Improve coverage of storing credentials in parser.Stack"
Merge "Add coverage for trusts parser.Stack delete path"