Initial groundwork for generating test mode Stripe Connect URLs

[james-allan]

Fixes #3291

Changes proposed in this Pull Request:

This PR updates the WC_Stripe_Connect_API class to add general support for generating test mode Stripe Connect URLs. It also updates the placeholder URL being used for the Connect a test account button on the Configure connection modal - it was previously using the live URL.

Testing instructions

1. Checkout this branch.
2. Run npm run build
3. Go to WooCommerce > Settings > Payments > Stripe > Settings (tab)
4. Click Configure connection to option the Stripe Connect modal.
5. Click on Test tab.
6. Click on Create or connect a test account button.
7. You should land on a Stripe connect page with test mode labels.

8. Clicking the Create or connect an account on the Live tab should still direct you to the live mode connect page.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[james-allan]

In the previous PR these strings weren't translated (🙈) and in this PR I've also taken the opportunity to make the button text consistent with the text on the other connect UI.

^{Create or connect an account button}

[diegocurbelo]

I added a button (secondary if there is a valid live url) to connect a test account:

[james-allan]

Oh I had been working on this test button over here: #3318

[james-allan]

In my PR I've taken into consideration the notice and the case if only 1 of the URLs is returned. I think we can probably merge this PR once it's good and then my PR can just be updated to improve on it.

[diegocurbelo]

Yeah, nice work on #3318, those error notices over the buttons look awesome.

[diegocurbelo]

The sandbox endpoints have been deployed to the connect server, we can merge this.

[diegocurbelo]

The code looks good, and it tests well... 🚢

[james-allan]

I'm going to merge this as there's an approving review and it's been a big PR that we've both worked on.

There is an issue we will need to fix however. I cannot onboard to a live account.

I suspect what is happening is we generate the oauth URL and store in a transient the state that is returned. Because we generate the test mode URL second, the test mode state overrides the live mode state and so when the user returns after on-boarding the state verification fails because it's comparing the live state returned with the test state stored in the transient. I'll follow up in another PR to fix that.

[diegocurbelo]

I'm going to merge this as there's an approving review and it's been a big PR that we've both worked on.

There is an issue we will need to fix however. I cannot onboard to a live account.

I suspect what is happening is we generate the oauth URL and store in a transient the state that is returned. Because we generate the test mode URL second, the test mode state overrides the live mode state and so when the user returns after on-boarding the state verification fails because it's comparing the live state returned with the test state stored in the transient. I'll follow up in another PR to fix that.

That seems like the likely cause, I'll fix it while adding the Stripe App flow.