Payment Modal does not close when using plugin with test invironments.

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• Made sure you’re on the latest version ✅ 1.9.14
• Used the search feature to ensure that the bug hasn’t been reported before ✅

Describe the bug
When using the plugin with our staging and test versions of the app, the pricing modal does not close after completing a purchase transaction.

To Reproduce
Steps to reproduce the behavior:

1. Point the app api URL to one of the test app URIs
2. Complete a test purchase in the plugin's pricing/checkout modal.
3. See the modal does not close after the transaction completes.

Expected behavior
The modal should close correctly in conjunction any app environment.

Additional Context
This issue is the only issue left from #558, which was trying to solve issues around the pricing modal that arose on-the-fly during the 2021 spring campaign.

Backlog Grooming (for WP Media dev team use only)

• Reproduce the problem
• Identify the root cause -- the test and development URIs are not set correctly in the pricing modal js.
• Scope a solution -- add the correct URIs into the checkPluginMessage() method in pricing-modal.js
• Estimate the effort: [XS]