UnionToIntersection: Fix incorrect test case

[som-sm]

The & Union bit in UnionToIntersection was added in #682, but the test case added along with it was incorrect.

type-fest/source/union-to-intersection.d.ts

Line 60 in 3bd9de6

? Intersection & Union

The test actually does nothing, as already pointed out in #682 (comment).

type-fest/test-d/union-to-intersection.ts

Lines 11 to 14 in 3bd9de6

	// It's possible to index by the resulting type.
	type ObjectsUnion = {a: string; z: string} | {b: string; z: string} | {c: string; z: string};
	declare const value: ObjectsUnion[UnionToIntersection<keyof ObjectsUnion>];
	expectType<string>(value);

This PR adds a generic assignability test that correctly verifies the & Union change as it only passes when the change is present. The updated test case basically verifies that UnionToIntersection<T | U | ...> is assignable to T | U | ....

Thanks to @Beraliv for pointing this out in ts-essentials/ts-essentials#451 (comment).

[Beraliv]

Thank you for responding so quickly, it makes sense to me now!