cmake: unittest: add `-t run` support

Add support to run unit tests directly from `west build`, for
environments where `west build -t run-test` (which runs the binary under
valgrind) is inappropriate or unavailable (WSL).

`west build -t run` also has the muscle-memory advantage of being the
same target name as the `native_sim` boards.

Signed-off-by: Jordan Yates <jordan@embeint.com>