We have had a couple reports of this, and I have found the current behavior very confusing. I think we need to do a better job of enforcing and diagnosing the conventions so users know the expected layout.

We can consider this problem to be fixed now. For one thing, the conventions are now more natural (in particular SwiftPM no longer silently appends a `TestSuite` suffix to the module name under `Tests` – rather, it expects the subdirectory name itself to have a `Tests` suffix, which is at least clearer.

And equally importantly, there are now decent error messages when a test suite doesn't have the right name:

sh> swift build
error: the module at Tests/abc has an invalid name ('abc'): the name of a test module has no ‘Tests’ suffix
fix: rename the module at ‘Tests/abc’ to have a ‘Tests’ suffix

(nit pick about the `fix` message: we're specifically not telling the user what to rename the test to by just appending `Tests` to what they already have, since the fact that a `Tests` suffix is needed might mean that they want to pick a different name altogether).