fix(virtual-scroll): move views that are already attached instead of inserting

[kara]

This PR does two things:

1. Fixes a minor issue in CdkVirtualScroll where sometimes views were inserted twice.

2. Fixes some tests that were relying on the fact that an externally requested ViewContainerRef would be the same instance as the injected ViewContainerRef instance (specifically spyOn methods were spying on a different instance). This is an implementation detail that is changed in Ivy. Also fixes these tests so that they aren't spying directly on ViewContainerRef method calls (as these are also called internally by ViewContainerRef itself and could change at any time).

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