Add some simple e2e tests for executing CLI plugins

To help with this add a bad plugin which produces invalid metadata and arrange
for it to be built in the e2e container.

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/cli-plugins/util_test.go

@@ -0,0 +1,24 @@
+package cliplugins
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"gotest.tools/fs"
+	"gotest.tools/icmd"
+)
+
+func prepare(t *testing.T) (func(args ...string) icmd.Cmd, func()) {
+	cfg := fs.NewDir(t, "plugin-test",
+		fs.WithFile("config.json", fmt.Sprintf(`{"cliPluginsExtraDirs": [%q]}`, os.Getenv("DOCKER_CLI_E2E_PLUGINS_EXTRA_DIRS"))),
+	)
+	run := func(args ...string) icmd.Cmd {
+		return icmd.Command("docker", append([]string{"--config", cfg.Path()}, args...)...)
+	}
+	cleanup := func() {
+		cfg.Remove()
+	}
+	return run, cleanup
+
+}