test(drip): assert promo code email injects the FOUNDER view var

PromoCodeEmail is the only drip mail that adds extra view data, now via the
contentData() hook. Nothing asserted the promoCode variable, so a regression in
that hook would go unnoticed. Assert the rendered content data is exactly
userName + promoCode => FOUNDER.

tests/Feature/MailSenderTest.php

@@ -117,6 +117,13 @@ test('only the paywall follow-up sets a reply-to, back to the drip sender', func
     expect((new WelcomeEmail($user))->envelope()->replyTo)->toBe([]);
 });
 
+test('promo code email adds the founder promo code to its view data', function () {
+    $user = User::factory()->create();
+
+    expect((new PromoCodeEmail($user))->content()->with)
+        ->toEqual(['userName' => $user->name, 'promoCode' => 'FOUNDER']);
+});
+
 test('default sender is used for active non-drip mailables', function (string $mailableClass) {
     $user = User::factory()->create();