restapi: Make mapper test more generic

The base class for mapper tests has specific bound for the "from" type
parameter:

  class AbstractInvertibleMappingTest<F extends BaseResource, T, I> ...

But this bound isn't used, and it makes this class unusable for things
other than resources. This patch removes that bound, so the class can be
used with resources and also with other simpler types:

  class AbstractInvertibleMappingTest<F, T, I> ...

Change-Id: I4176cd3a5d7b55ab7cbf00389d5b30a720da66f4
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>