fn: add Guard test helper

Copied from lightninglabs/loop

fn/tests.go

@@ -0,0 +1,51 @@
+package fn
+
+import (
+	"os"
+	"runtime/pprof"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// GuardConfig stores options for Guard function.
+type GuardConfig struct {
+	timeout time.Duration
+}
+
+// GuardOption is an option for Guard function.
+type GuardOption func(*GuardConfig)
+
+// WithGuardTimeout sets timeout for the guard. Default is 5s.
+func WithGuardTimeout(timeout time.Duration) GuardOption {
+	return func(c *GuardConfig) {
+		c.timeout = timeout
+	}
+}
+
+// GuardTest implements a test level timeout.
+func GuardTest(t *testing.T, opts ...GuardOption) func() {
+	cfg := GuardConfig{
+		timeout: 5 * time.Second,
+	}
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+
+	done := make(chan struct{})
+	go func() {
+		select {
+		case <-time.After(cfg.timeout):
+			err := pprof.Lookup("goroutine").WriteTo(os.Stdout, 1)
+			require.NoError(t, err)
+			panic("test timeout")
+
+		case <-done:
+		}
+	}()
+
+	return func() {
+		close(done)
+	}
+}