add regression test to its own file

app_regression_test.go

@@ -0,0 +1,29 @@
+package cli
+
+import (
+	"testing"
+)
+
+// TestRegression tests a regression that was merged between versions 1.20.0 and 1.21.0
+// The included app.Run line worked in 1.20.0, and then was broken in 1.21.0.
+func TestRegression(t *testing.T) {
+	// setup
+	app := NewApp()
+	app.Commands = []Command{{
+		Name: "command",
+		Flags: []Flag{
+			StringFlag{
+				Name: "flagone",
+			},
+		},
+		Action: func(c *Context) error { return nil },
+	}}
+
+	// logic under test
+	err := app.Run([]string{"cli", "command", "--flagone", "flagvalue", "docker", "image", "ls", "--no-trunc"})
+
+	// assertions
+	if err != nil {
+		t.Errorf("did not expected an error, but there was one: %s", err)
+	}
+}