XCTest is not fully open-source, only the Swift library parts. We don't have a
command-line runner available, so disable support.

--- a/Sources/Commands/Utilities/TestingSupport.swift
+++ b/Sources/Commands/Utilities/TestingSupport.swift
@@ -105,7 +105,7 @@
     ) throws -> [TestSuite] {
         // Run the correct tool.
         var args = [String]()
-        #if os(macOS)
+        #if false
         let data: String = try withTemporaryFile { tempFile in
             args = [try Self.xctestHelperPath(swiftTool: swiftTool).pathString, path.pathString, tempFile.path.pathString]
             var env = try Self.constructTestEnvironment(