e2e: test etcdctl v2 double user grant

Crashes in 2.3.1

e2e/ctl_v2_test.go

@@ -181,6 +181,12 @@ func testCtlV2GetRoleUser(t *testing.T, cfg *etcdProcessClusterConfig) {
 	if err := etcdctlUserGet(epc, "username"); err != nil {
 		t.Fatalf("failed to get user (%v)", err)
 	}
+	// ensure double grant gives an error; was crashing in 2.3.1
+	regrantArgs := etcdctlPrefixArgs(epc)
+	regrantArgs = append(regrantArgs, "user", "grant", "--roles", "foo", "username")
+	if err := spawnWithExpect(regrantArgs, "duplicate"); err != nil {
+		t.Fatalf("missing duplicate error on double grant role (%v)", err)
+	}
 }
 
 func TestCtlV2UserList(t *testing.T) {