Add delegation and targetpath search tests

[jku]

This is a bit tricky to test but we should ensure that multiple levels of targets delegations work correctly and that target path search never finds incorrect artifacts.

python-tuf has some parametrized tests in https://github.com/theupdateframework/python-tuf/blob/develop/tests/test_updater_delegation_graphs.py: TestDelegationsGraphs for the delegation tests and then TestTargetFileSearch for targetpath searching. They are a bit complicated but I don't really see better ways.

[jku]

the tests could be far more extensive but a first set is in there