Fix warnings and errors in tests

Author: MaxDesiatov

Sources/WAT/Location.swift

@@ -1,5 +1,5 @@
 /// A location in a WAT source file.
-public struct Location: Equatable, CustomDebugStringConvertible {
+public struct Location: Equatable, CustomDebugStringConvertible, Sendable {
     let index: Lexer.Index
     let source: String.UnicodeScalarView
 

Sources/WIT/Diagnostics.swift

@@ -6,7 +6,7 @@ struct DiagnosticError: Error {
     let diagnostic: Diagnostic
 }
 
-public struct Diagnostic {
+public struct Diagnostic: Sendable {
 
     public let message: String
     var textRange: TextRange?

Tests/WasmKitTests/Spectest/Spectest.swift

@@ -48,11 +48,11 @@ struct NullSpectestProgressReporter: SpectestProgressReporter {
 
 struct StderrSpectestProgressReporter: SpectestProgressReporter {
     func log(_ message: String, verbose: Bool) {
-        fputs(message + "\n", stderr)
+        fputs(message + "\n", FileHandle.standardError.fileDescriptor)
     }
     func log(_ message: String, path: String, location: Location, verbose: Bool) {
         let (line, _) = location.computeLineAndColumn()
-        fputs("\(path):\(line): " + message + "\n", stderr)
+        fputs("\(path):\(line): " + message + "\n", FileHandle.standardError.fileDescriptor)
     }
 }