Remove extraneous @escaping and @Sendable attributes from CustomExecutionTrait.execute(...) SPI

[stmontgomery]

This refines the CustomExecutionTrait SPI protocol by removing two unnecessary attributes from the signature of its execute(_:for:testCase:) method requirement:

1. The @escaping attribute on the function parameter was needed originally to avoid a warning, but function should not actually be considered escaping and part of the contract of this interface is that the implementor must call it before returning. The compiler no longer complains about this, so we can enforce this requirement by removing the unnecessary @escaping attribute now.
2. The outer @Sendable was necessary when this SPI was first written to avoid a Swift concurrency warning, since a reference to the function is taken elsewhere in the codebase, but this is no longer needed in recent Swift compilers.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[grynspan]

This is technically ABI-breaking, but it's also experimental SPI so it's fine.