add dumpInertia() testing method

[JulienCauvin]

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This pull request intent is to add a new dumpInertia(string $key = null) method to TestResponseMacros file. Existing testing methods like dump(), dumpSession(), dumpHeaders() are very convenient in Laravel testing.

If parameter is not set, then all of the Inertia data will be printed (component, props, url, version, ...). If parameter is set, then props property content will be printed (accepting dot-notation thanks to Arr:get())