error: no such module 'XCTest'

Hi, Was looking into this library for a project and got some errors running the tests locally.

### Steps to Reproduce
```
git clone https://github.com/apple/swift-binary-parsing
git checkout 0.0.1
swift test
```

```
macOS: 26.1 (25B78)

swift --version
Apple Swift version 6.2.1 (swift-6.2.1-RELEASE)
Target: arm64-apple-macosx26.0
Build config: +assertions
```

### Expected behavior
Passing tests for release branch.

### Actual behavior
```
swift-snapshot-testing/Sources/SnapshotTesting/AssertSnapshot.swift:1:8: error: no such module 'XCTest'
1 | import XCTest
| `- error: no such module 'XCTest'
2 |
3 | #if canImport(Testing)
```

Thanks,
Zane

Reproduce the failure by checking out tag 0.0.1 and running swift test. Start with the XCTest import error in swift-snapshot-testing/Sources/SnapshotTesting/AssertSnapshot.swift, then trace why XCTest is unavailable for this test setup. Done means the release branch tests pass on the reported Swift and macOS environment.

Written by the indexing model from the issue text.