Arcyd: branchtester: cover branch and branchmock

Update abdt_branch__t and abdt_branchmock__t to use abdt_branchtester.

Move branch status assertions to abdt_branchtester.

Fix BranchMock contract violations.

Test Plan: $ ./precommit.sh
Add "system tests" to precommit

Add the existing 'exercise_arcyon.sh' script to the suite of things to
run before committing.

Move 'runtime_tests.sh' -> 'unit_tests.sh'

More 'progress' output from tests.

Test Plan: $ ./precommit.sh
Manually introduce errors to make sure they're reported by
system_tests.sh
WIP