Makefile: rename clar-related variables to avoid confusion

Makefile: rename clar-related variables to avoid confusion

The Makefile variables related to the recently-introduced clar testing
framework have a `UNIT_TESTS_` prefix. This prefix is extremely similar
to the prefix used by our other unit tests that use our homegrown unit
testing framework, which is `UNIT_TEST_`. The consequence is that it is
easy to misread the names and confuse them with each other.

Rename the clar-related variables to instead have a `CLAR_TEST_` prefix
to address this.

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