Issue Description:

It appears that XCTestCase.expectation(description: "…") calls through to XCTestCase.addExpectation(…) which asserts that it is being called from the main thread. See

When running in an async test case, the test case is not always run on the main thread, so the test fails and crashes the running suite due to this assertion.

A simple test case is to call the following code within an async test, something like:

class MyTests: XCTestCase {
    func testAThing() async throws {
        let expectation = expectation(description: "Sending email")
    }
}

And then run it on Linux using any recent Swift 5.5 snapshot.

The same code runs fine under macOS under Xcode 13.2 beta 2, so this appears to be a difference between Swift's implementation of XCTest and Xcode's.