swift234

[docs] Recommend using utils/build-script to test

Contributors likely won't need to invoke CMake directly in order to run the test suite. As such, the current testing documentation is a little misleading: it jumps into a detailed explanation of the `check-swift` family of targets, even though those are abstracted away via the build script. For example, I remember when I first read this document, I spent 20 minutes searching in vain for a build executable named `check-swift`.

Instead, change the wording to make it clear that `utils/build-script` is the best way to run the tests. For those interested in **how** the tests are executed, show an example of a CMake command that runs them.