[Bug]: Building is broken on main due to 'error: no such module Testing'

### I have done the following

- [x] I have searched the existing issues
- [x] If possible, I've reproduced the issue using the 'main' branch of this project

### Steps to reproduce

To reproduce:

1. Create a fresh clone of the container repository
2. Run `make all`

### Problem description

```
/Users/me/code/apple-container/Sources/ContainerTestSupport/BuildFixture.swift:21:8: error: no such module 'Testing'
19 | import Foundation
20 | import SystemPackage
21 | import Testing
| `- error: no such module 'Testing'
22 |
23 | // MARK: - Build context types
```

### Environment

```markdown
- OS: Tahoe 26.4.1
- Xcode: 26.4.1.0.1775747724
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with a fresh clone and run `make all` to reproduce the failure, then inspect `Sources/ContainerTestSupport/BuildFixture.swift` and the project’s build configuration around the `Testing` import. Done means the project builds successfully on the reported environment without the “no such module 'Testing'” error.

Written by the indexing model from the issue text.