Add testing script (with SPM workaround) and update CONTRIBUTING.md

Author: stackotter

CONTRIBUTING.md

@@ -6,6 +6,10 @@
 2. Install the required dependencies as detailed in the [readme](README.md)
 3. Open Package.swift to open the package in Xcode and you're ready to code, have fun
 
+## Running tests
+
+Due to limitations of Swift Package Manager and the way this project is structured, running tests is a little more annoying than just running `swift test`. Luckily, there's a handy [test.sh](test.sh) script which performs the required workaround. To run the tests, just run `./test.sh` in the root of the repository.
+
 ## How to do something useful
 
 1. Look through [the issues on GitHub](https://github.com/stackotter/swift-cross-ui/issues) and

test.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+# `swift test` builds all targets in the package (even those not depended upon
+# by any test targets), which leads to `swift test` on its own being broken
+# for SwiftCrossUI
+swift test --test-product swift-cross-uiPackageTests