next-checks: add clippy-test to next-clippy-checks

We are currently only running clippy checks for each platform but
not for the tests.

 * Run clippy checks for all platforms and for tests. The latter is
   done via the new "clippy-tests" target that was recently added
   on the Makefile.

 * Remove repetition of clippy tests on the platforms already set up

Change-Id: I5558c77e942cf4ada0d4bc721315f4fcf289ab18
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
3 files changed