hevm: fix `--verbose`

The `--verbose` flag is kinda weird and has inconcistent semantics between the various test types at the moment. Ideally we would have two verbosity levels:

1. show traces & log output for failed tests
2. show traces & log output for all tests

For passing symbolic tests we want to show the full behaviour tree (as with `--show-tree`), for failing symbolic tests we can show the specific trace that triggered the assertion violation.

For passing fuzz tests we can either show 0 traces, or `--fuzz-runs` traces, the later is probably impractical. For failing fuzz tests we can show the specific trace that triggered the assertion violation.

For concrete tests we always have a single canoncial trace to show in both pass and fail cases.