Move MockStore tests to a separate file

[timdeschryver]

Currently the MockStore tests are added to the Store tests, this makes them harder to find. That's why we want to extract the MockStore tests at https://github.com/ngrx/platform/blob/master/modules/store/spec/store.spec.ts#L450 to their own file.

[timdeschryver]

This one is picked up by @deli6z.

[alex-okrushko]

I didn't realize this issue was opened - I moved the tests as part of #1878

[evgenyfedorenko]

@alex-okrushko stole my chance to shine.

[brandonroberts]

Fixed via #1878

[alex-okrushko]

@alex-okrushko stole my chance to shine.

@deli6z there's plenty more! :)