Add tests for `[BindingBehavior]` applied to page model properties

[dougbu]

Found while verifying #6811.

Though IsBindingAllowed and IsBindingRequired are likely now set correctly in ModelMetadata -- as part of the #6790 fixes, [BindRequired], [BindNever] and so on do not impact model binding of page model properties.

For example

[BindProperty(SupportsGet = false)]
[BindRequired]
public string Required { get; set; }

works as expected if the Required property is in a controller. But no model error occurs when the same property is in page model. (In both cases, the form does not include an <input/> field for Required.)

[dougbu]

Note this issue basically means #6811 was not fixed.

[mkArtakMSFT]

@dougbu, looking through the details of the referenced issue it seems it was closed intentionally, without any action as that was called out as the best approach - to do nothing.
@pranavkm, is this a valid exception, or this should still be acted on so we get to consistency between Controllers and Razor Pages?

[pranavkm]

Might be a dup of #7358, although it would be good to verify this and add tests to cover this specific scenario.

[dougbu]

#7358 is about [Validation] subclasses while this is about [BindRequired]. It's still possible fixing #7358 fixed these too. Or, another of our changes in this area may have fixed [BindRequired] in page model scenarios.

Should check if these symptoms occur with Preview 2 bits.

[mkArtakMSFT]

@kichalla, can you please evaluate whether this is still the case, and whether there is still any action here or not.

[pranavkm]

This was addressed as part of #7358. What it does need is a test or two.