Update keyof tests to reflect #12425

[ethanresnick]

#12425 removed number from the union type constituting keyof T, but the comments in the test cases still had number as a possible type. This PR changes the comments in the tests so they match the actual behavior.

It also adds a test that numeric index signatures:

keyof NumericallyIndexed<Shape> // checks that this is never type NumericallyIndexed<T> ={[x: number]: T }; 

[msftclas]

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[mhegazy]

thanks!