Add option to specify when we expect a counterexample

[karmacoma]

Currently:

• tests with no counterexamples return 0 and a green [PASS]
• tests with counterexamples, or unknown results return non-0 and a red [FAIL]

This makes running the test suite a little scary, because we see a mix of [PASS] and [FAIL] but sometimes [PASS] can be bad (if we expected a counterexample) and [FAIL] can be bad (if we didn't expect a counterexample).

We should add the --expect-counterexample option (open to better naming suggestions):

• tests that do have 1 or more counterexamples return exitcode 0 and a green [XFAIL] (for "expected fail" -- we found the thing we're looking for)
• tests that fail with no counterexamples are still a red [FAIL] (it fails for unexpected reasons), they return exitcode != 0
• tests that would normally be a green [PASS] are now a red [XPASS] (for "unexpected pass" -- we didn't find the thing we're looking for), they return exitcode != 0

Similarly, we should have an --expect-fail option that behaves similarly but for tests that fail with no counterexamples.

Then we can add the right /// @custom:halmos --expect-counterexample annotations to solidity tests and running halmos --root tests should output a mix of green [PASS] and [XFAIL] and return 0