Self-document integration test mode

[Janpot]

• Rename TOOLPAD_INTEGRATION_TEST to TOOLPAD_ENABLE_CREATE_BY_DOM: This signals the exact purpose to maintainers, to prevent abusing it for other purposes. e.g. to make a test pass.
• This also allows us to configure our PR previews with this variable to make it easier to test production setups on PR previews.
• Throw a meaningful error when integration tests are run when this variable is not set. (Allow duplicating apps #658 (comment))
• Make sure all integration tests use the same code path to create their applications, so they perform the check of this PR.

[render]

Your Render PR Server URL is https://toolpad-pr-995.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-ccgsamha6gdul87a4t70.

[bharatkashyap]

Got it!