Additionally when I try to build tests in a docker container with image swiftlang/swift:nightly-5.5-focal, I get errors such as this:
/build/.build/x86_64-unknown-linux-gnu/debug/my-async-project-serverPackageTests.derived/AppTests.swift:109:29: error: invalid conversion from 'async' function of type '() async throws -> ()' to synchronous function type '() throws -> Void' testCase(BasicAsyncTests.__allTests__BasicAsyncTests),
As long as you stay out of the Xcode environment. XCTest is provided as private frameworks and plugins within Xcode, and it supports all the languages that Xcode supports (C/C++/Obj-C/Obj-C++/Swift), not just Swift.
If you are writing for Linux, Windows, etc., shouldn't have a problem.