tests: remove repetition in unit test object deps

tests: remove repetition in unit test object deps

Most of the unit tests have identical sets of object deps.
For example all block unit tests need to depend on

 $(block-obj-y) libqemuutil.a libqemustub.a

Currently each unit test repeats this list of test deps.
This list of deps will grow as future patches add more
modules to the build, so define some common variables
that can be used by all unit tests to remove the
repetition.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>