This moves a lot of the test support code into its own package which is only imported for tests,
so it's not shipped as a part of core Corda. The node currently depends on this support code to
compile, although future work could try to separate this out. This change highlights that parts
of production code is dependent on test elements (i.e. dummy keys), and makes it harder for
such accidental crosses to occur later.
An integration test category is also added as part of this work, to contribute towards COR-345.
