[spm] Fix for Swift 6.0 compatibility

Swift 6.1 (Xcode 16.3+) adds broader support for trailing commas in
function call argument lists (see SE-0439), but Swift 6.0 is stricter.
Package.swift used a trailing comma after the last argument in a
`.testTarget()` call, which is invalid syntax for that version.
This change removes the trailing comma to ensure
the package manifest can be parsed correctly by Swift 6.0.