Print additional details when emitting assertion failure messages

[briancroom]

This brings the output in line with what Darwin's XCTest outputs

Take a failing assertion like:

func testAssertEqual() {
    XCTAssertEqual(1, 2, "message", file: "test.swift")
}

The current behavior of swift-corelibs-xctest is to produce a failure message like:

test.swift:36: error: Test.testAssertEqual : message

Darwin's XCTest produces:

test.swift:36: error: -[Test.Test testAssertEqual] : XCTAssertEqual failed: ("Optional(1)") is not equal to ("Optional(2)") - message

After this patch, the output from swift-corelibs-xctest would be:

test.swift:36: error: Test.testAssertEqual : XCTAssertEqual failed: ("Optional(1)") is not equal to ("Optional(2)") - message

Note that this now includes the name of the failing assertion function and includes details about the asserted expressions when appropriate.

This change just screams for test coverage, but I guess that will have to wait until #20 or similar gets merged. In the meantime, though, here is a test fixture that I wrote while working on this, and have been using to manually verify the new behavior.

[ddunbar]

It would be great to get this in, the current failure messages are very unfortunate when we see CI failures using a Linux XCTest.

[parkera]

This looks good to me. @mike-ferris-apple should take a look too.

[briancroom]

I've just rebased this now.

[mike-ferris]

This looks good and some testing with Xcode's XCTest shows that the output is not almost the same for these. The one exception seems to be XCTAssertNil where you have the failureDetails returning ""not nil"". To get the same output as Xcode's XCTest, this should return nil. Can you make that tweak? Then I'll definitely merge this! Thanks for providing this, it will make the output a ton more useful.

[briancroom]

Good catch @mike-ferris-apple! I think I confused myself by choosing "notnil" as the object in the fixture I was working off of. 😁

I went back to closely compare the output from Xcode's XCTest again, and realized that does include the actual value of the expression in the message, e.g.:

test.swift:80: error: -[Test.Test testAssertNil] : XCTAssertNil failed: "MyString" - message

I have updated the PR to properly capture and emit this expression value in the failure message.

[mike-ferris]

Thanks!