Avoid using `String.init(contentsOfFile:encoding:)` in `PerformanceTests.swift`
Avoid using `String.init(contentsOfFile:encoding:)` in `PerformanceTests.swift`

`String` created by `String.init(contentsOfFile:encoding:)` can’t use `String.UTF8View.withContiguousStorageIfAvailable(_:)` on Apple Platform.

jpsim/Yams,jpsim/Yams,jpsim/Yams