Add spec for StripeSCA payment gateway

We didn't actually have a unit test for this.

spec/models/spree/gateway/stripe_sca_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Spree::Gateway::StripeSCA, type: :model do
+  before { allow(Stripe).to receive(:api_key) { "sk_test_12345" } }
+
+  describe "#purchase" do
+    let(:order) { create(:order_with_totals_and_distribution) }
+    let(:credit_card) { create(:credit_card) }
+    let(:payment) {
+      create(
+        :payment,
+        state: "checkout",
+        order: order,
+        amount: order.total,
+        payment_method: subject,
+        source: credit_card,
+      )
+    }
+    let(:gateway_options) {
+      { order_id: order.number }
+    }
+    let(:payment_authorised) {
+      payment_intent(payment.amount, "requires_capture")
+    }
+    let(:capture_successful) {
+      payment_intent(payment.amount, "succeeded")
+    }
+
+    it "captures the payment" do
+      stub_request(:get, "https://api.stripe.com/v1/payment_intents/12345").
+        to_return(status: 200, body: payment_authorised)
+      stub_request(:post, "https://api.stripe.com/v1/payment_intents/12345/capture").
+        with(body: {"amount_to_capture" => "10.0"}).
+        to_return(status: 200, body: capture_successful)
+
+      response = subject.purchase(order.total, credit_card, gateway_options)
+
+      expect(response.success?).to eq true
+    end
+  end
+
+  def payment_intent(amount, status)
+    JSON.generate(
+      object: "payment_intent",
+      amount: amount,
+      status: status,
+      charges: { data: [{ id: "ch_1234", amount: amount }] }
+    )
+  end
+end