Make MacroTestingConfiguration Sendable

[bonkey]

In my package for 5.10, with Xcode 16.2 I was getting that, so why not sendablize it?

.../SourcePackages/checkouts/swift-macro-testing/Sources/MacroTesting/MacrosTestTrait.swift:11:9: error: stored property 'configuration' of 'Sendable'-conforming struct '_MacrosTestTrait' has non-sendable type 'MacroTestingConfiguration'
    let configuration: MacroTestingConfiguration
        ^
.../SourcePackages/checkouts/swift-macro-testing/Sources/MacroTesting/AssertMacro.swift:770:8: note: consider making struct 'MacroTestingConfiguration' conform to the 'Sendable' protocol
struct MacroTestingConfiguration {
       ^
                                 : Sendable