Split the hs-check test rule

Currently, both the unit test and the shell tests are run (manually)
from this rule. This has two downsides: you can't (compile and) run
just one of them easily, and make can't run them in parallel.

By splitting the rules, we can both of the above easily.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>