Add @_spi annotations to reexport decls in cross-import overlays. (#870)

This PR attempts to make sure that SPI is reexported from our
cross-import overlays. This change is speculative and I'm not actually
sure if it'll do what I want.

### 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: grynspan

Sources/Overlays/_Testing_CoreGraphics/ReexportTesting.swift

@@ -8,4 +8,4 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@_exported public import Testing
+@_exported @_spi(Experimental) @_spi(ForToolsIntegrationOnly) public import Testing

Sources/Overlays/_Testing_Foundation/ReexportTesting.swift

@@ -8,4 +8,4 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@_exported public import Testing
+@_exported @_spi(Experimental) @_spi(ForToolsIntegrationOnly) public import Testing