Add sometimes assertions

[0xalpharush]

A common issue is not knowing whether some state is being reached effectively and wanting greater insight into execution statistics (see this issue #431 and PR with some progress here #364)

We can add "tests" that fail if they are not successful some percentage of the time (TBD if configuring this is desirable) by making the fuzzer monitor the success/fail rate. By encouraging a pattern like the following, they should get put in the corpus, but we could consider prioritizing them as well.

function fuzz() external {
bool result = ...
emit sometimesTrue(result); // fail if never true
....
}

function sometimesSucceeds() external { // fail if always reverts
... 
}

This is effectively asserting that some path of the code is reached. Some test harnesses/fuzzers will let you mark a line of code as expected to be reached by a given input, for example. That way it is clear the harness/test was not invalidated or regressed to a point that it's no longer effective thereby serving as a "fuzz canary", alerting devs of trouble in the mines.

See also https://antithesis.com/docs/best_practices/sometimes_assertions.html

[bsamuels453]

crytic/echidna#880