unit tests: add rule for running with test-tool

unit tests: add rule for running with test-tool

In the previous commit, we added support in test-tool for running
collections of unit tests. Now, add rules in t/Makefile for running in
this way.

This new rule can be executed from the top-level Makefile via
`make DEFAULT_UNIT_TEST_TARGET=unit-tests-test-tool unit-tests`, or by
setting DEFAULT_UNIT_TEST_TARGET in config.mak.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>