[SR-5262] XCTest should have a way to validate assertions (e.g. `assert()`)

[phausler]

Previous ID	SR-5262
Radar	None
Original Reporter	@phausler
Type	New Feature

Additional Detail from JIRA

Votes	0
Component/s	XCTest
Labels	New Feature
Assignee	None
Priority	Medium

md5: 45492463837a5115b54a5e4cdffbc273

Issue Description:

Often times when testing code it is helpful to validate assertion cases that have preconditions or fatalErrors. A method that would aide in this endeavor would be something along the lines of XCTAssertIsFatal where a block of code is passed in that is expected to trigger a fatalError.

Normally this would mean that the code would crash. However if a signal handler for SIGILL is installed we can validate if an assertion is tripped.

See https://github.com/phausler/swift-corelibs-xctest/tree/assert_fatal for a rough draft at handling fatal assertions (note this only works in a non attached debugger mode and has only been written for Darwin, however a linux port should be trivial)

[belkadan]

This has been discussed before. StdlibUnittest takes a different approach: fork off a child process, and check its output. That's a little less dangerous—recovering from a fatalError means leaking memory at the very least.