Mark `fail()` function `virtual`

[0xbok]

When writing tests and experimenting, sometimes it is beneficial to let fail() quit as soon as the first test fails. This helps debugging and avoid a lot of logs on screen.

Something like this

function fail() internal {
     revert();
}

Marking this function virtual allows us to update this function as necessary.

[PaulRBerg]

As discussed in #30, one of the goals of DSTest is to let users run multiple assertions in one test.

If the fail function reverted, or was made virtual, the assumptions made by multiple dependees (such as Foundry) would break.

[0xPhaze]

I'm in favor of this change. A dev might have a reason/need to explicitly override this function. Being able to change the fail() behavior to an assert(false) statement will make compatibility with other frameworks, such as Echidna and the SMTChecker much easier.

[PaulRBerg]

@0xPhaze if you'd like to get immediate support for a virtual fail() function today, check out PRBTest. It's completely compatible with Forge Std, e.g. see how I use it in PRBMath.

[0xPhaze]

Great, good to know. I'll use it for now. I'd still like to see this added to ds-test in the future.