Test runner: allow automatic instantiation of intermediate services

[ljacqu]

Current test runner is very strict with @InjectDelayed fields in that if a dependency is not declared, it fails. It would however be nice to allow intermediate dependencies to be instantiated (as real dependencies) for (partial) integration tests. Relates to #28

Inform PerWorldInventory project when done; a test is currently blocked by this https://github.com/Gnat008/PerWorldInventory/blob/master/src/test/java/me/gnat008/perworldinventory/data/FileWriterTest.java