🏗 Ban private properties in integration tests

[jridgewell]

Re: https://github.com/ampproject/amphtml/issues/24124

[mrjoro]

Is this ready to merge in?

[kristoferbaxter]

Looks like this PR doesn't address all of the use-case since some integration tests appear to run only when non-obfuscated.

For instance: https://github.com/ampproject/amphtml/blob/master/test/integration/test-video-players-helper.js#L65

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