Don't trigger MessageBlobStore during tests

Don't trigger MessageBlobStore during tests

The test for OutputPage::makeResourceLoaderLink was triggering database
queries through MessageBlobStore even though it doesn't use any
messages.

In bb03d1a8e08 I had made MessageBlobStore a singleton instead of static
functions, however there's no need for it to be one since the class is
stateless. Callers can just create a new MessageBlobStore instance and
call functions upon it. Using getInstance() is now deprecated.

ResourceLoader now has a setMessageBlobStore setter to allow overriding
which MessageBlobStore instance will be used. OutputPageTest uses this
to set a NullMessageBlobStore, which makes no database queries.

Change-Id: Ica7436fb6f1ea59bd445b02527829ab0742c0842