t/unit-tests: rename clar-based unit tests to have a common prefix

t/unit-tests: rename clar-based unit tests to have a common prefix

All of the code files for unit tests using the self-grown unit testing
framework have a "t-" prefix to their name. This makes it easy to
identify them and use globbing in our Makefile and in other places. On
the other hand though, our clar-based unit tests have no prefix at all
and thus cannot easily be discerned from other files in the unit test
directory.

Introduce a new "u-" prefix for clar-based unit tests. This prefix will
be used in a subsequent commit to easily identify such tests.

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