Assign "Test Scoping Traits" proposal a number (#900)

Assign the "Test Scoping Traits" proposal, which was recently approved
and merged in #733, a number

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Documentation/Proposals/NNNN-custom-test-execution-traits.md renamed to Documentation/Proposals/0007-test-scoping-traits.md

@@ -1,6 +1,6 @@
 # Test Scoping Traits
 
-* Proposal: [SWT-NNNN](NNNN-filename.md)
+* Proposal: [SWT-0007](0007-test-scoping-traits.md)
 * Authors: [Stuart Montgomery](https://github.com/stmontgomery)
 * Status: **Awaiting review**
 * Implementation: [swiftlang/swift-testing#733](https://github.com/swiftlang/swift-testing/pull/733), [swiftlang/swift-testing#86](https://github.com/swiftlang/swift-testing/pull/86)