Add flag to nargo test to ignore failures due to missing foreign calls

[TomAFrench]

For repositories such as aztec-packages there's a number of tests which rely on foreign call handlers, however we want to be able to run tests on the remainder of functions to see if any failures occur in pure Noir functions.

We should then add a flag which tells nargo to ignore any of these functions and mark them as skipped. For now we can just run the test and catch any unhandled foreign calls so there's no need to inspect the functions beforehand.

[TomAFrench]

It's kinda annoying to pull out that a given test has failed due to an unhandled foreign call. I think the cleanest way to go about this is to have a specified foreign call handler for tests which tracks whether it's encountered an unknown foreign call and then we can modify the test status based off of that.

I'm doing some work to make the foreign call handlers more modular so we don't keep making the default one more and more complex.