test: add resumable isolated test harness

test/harness/lock_windows.go

@@ -0,0 +1,19 @@
+//go:build windows
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func acquireLock(path string) (func(), error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
+	if err != nil {
+		return nil, fmt.Errorf("another test harness process may hold %s: %w", path, err)
+	}
+	return func() {
+		_ = file.Close()
+		_ = os.Remove(path)
+	}, nil
+}