resourceloader: Improve test cases for MessageBlobStore

resourceloader: Improve test cases for MessageBlobStore

Move source code to includes/resourceloader to match
test case. This is part of ResourceLoader and not meant
to be used elsewhere.

Merge two similar test cases for getting blobs and fetching
messages which were doing the same thing.

Rewrite the test names to be a better reflection of the stories
they test, add comments for why, and re-order them to put related
tests together.

Move test-utilities to the bottom and make them actually private.

Change-Id: I7a437eebf3ba6a722e286dfe77c2f9fe49ad222f