signal checker extensions

[eschmidscs]

This PR contains a bunch of changes that I missed.

• Have it in the context, just for usability
• Allow to disable the monitoring. We have lots of error signals monitored by this block. During reset, these signals do about anything. It becomes quite tedious to add all steps to simulation start...
• Function to actually read back the monitored value. A TB might just read the value for further actions.
• Add a checker_t as well. The logger_t does not add to the checker statistics.

[eschmidscs]

Sorry, too fast here: I have to update the TB.

[eschmidscs]

At least I did not have the disabled logs in active memory... Having an enable_logs might work, yes. If the test_runner would not fail on the disabled logs, but still fail on the later, enabled logs.