Prevent blackbox_test_runner from passing without invariant assertions

[michaeldiamant]

Adds an assertion to confirm blackbox_test_runner tests supply at least 1 invariant. The intent is to prevent a test from passing when it either does not declare invariants or structures invariant declaration incorrectly.

Notes:

• While reviewing Support local variables with frame pointers #559, I discovered that no invariant assertions happen in test_blackbox_subroutines_as_apps and test_blackbox_subroutines_as_logic_sigs because the tests configure invariants with key = assertions while blackbox_test_runner looks for key = invariants.
• Changing the test keys from assertions to invariants results in failed tests. I open the PR to request help in fixing the broken assertions. Once fixed, I'd like to add the defensive check shown here.
• Reviewing git history, I confirmed the key mismatch exists since the functionality was originally merged in Semantic Blackbox Testing #249.

[michaeldiamant]

Closing - The defensive check will be brought in via #605.