Add additional amp-sidebar validator tests

[dknecht]

This test checks the number of children requirement

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[erwinmombay]

/to @powdercloud

[dknecht]

@powdercloud PTAL

[powdercloud]

It seems this is a test for amp-accordion > section, and not all that specific to amp-sidebar.
Could this fit into here, maybe?
https://github.com/ampproject/amphtml/blob/master/extensions/amp-accordion/0.1/test/validator-amp-accordion.html

[dknecht]

Yes. I will move it there.

[dknecht]

Can we merge this?

[jridgewell]

@powdercloud?