Makefile: stop listing test library objects twice

Makefile: stop listing test library objects twice

Whenever one adds another test library compilation unit one has to wire
it up twice in the Makefile: once to append it to `UNIT_TEST_OBJS`, and
once to append it to the `UNIT_TEST_PROGS` target. Ideally, we'd just
reuse the `UNIT_TEST_OBJS` variable in the target so that we can avoid
the duplication. But it also contains all the objects for our test
programs, each of which contains a `cmd_main()`, and thus we cannot link
them all into the target executable.

Refactor the code such that `UNIT_TEST_OBJS` does not contain the unit
test program objects anymore, which we can instead manually append to
the `OBJECTS` variable. Like this, the former variable now only contains
objects for test libraries and can thus be reused.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>