test: add resumable isolated test harness

test/harness/lock_unix.go

@@ -0,0 +1,24 @@
+//go:build !windows
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+func acquireLock(path string) (func(), error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	if err := syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+		_ = file.Close()
+		return nil, fmt.Errorf("another test harness process holds %s: %w", path, err)
+	}
+	return func() {
+		_ = syscall.Flock(int(file.Fd()), syscall.LOCK_UN)
+		_ = file.Close()
+	}, nil
+}