Constrained tests tool implementation + skill (#5873)

* constrained tests tool implementation + skill

* pr suggestions

Tools/Sources/Commands/CI/CPUConstraint.swift

@@ -0,0 +1,40 @@
+import Foundation
+
+/// Saturates the CPU with busy processes for the duration of a test run, approximating
+/// a loaded CI runner where timing-sensitive tests are starved of processor time.
+final class CPUConstraint {
+    private var processes = [Process]()
+    
+    /// 2× the machine's cores is enough to keep the scheduler permanently oversubscribed.
+    private let taskCount = ProcessInfo.processInfo.processorCount * 2
+    
+    func start() {
+        logger.info("\n🐌 Constraining the CPU with \(taskCount) tasks…\n")
+        
+        for _ in 0..<taskCount {
+            let process = Process()
+            process.executableURL = URL(filePath: "/usr/bin/yes")
+            process.standardOutput = FileHandle.nullDevice
+            process.standardError = FileHandle.nullDevice
+            
+            do {
+                try process.run()
+                processes.append(process)
+            } catch {
+                logger.error("Failed to start a CPU constraining task: \(error)")
+            }
+        }
+    }
+    
+    func stop() {
+        guard !processes.isEmpty else { return }
+        
+        logger.info("\n🐌 Releasing the CPU…\n")
+        processes.forEach { $0.terminate() }
+        processes.removeAll()
+    }
+    
+    deinit {
+        stop()
+    }
+}