[ty] Update tests `reveal_type` and `Never` (#23418)
## Summary
`reveal_type(something_of_type_never)` is a call to a function that
returns `Never`, since `reveal_type` returns the type of its argument.
So it is conceivable that we treat it just like any other call to a
terminal function in terms of control flow analysis. This change to our
tests prepares us for that scenario in that it splits tests into
multiple functions, so that assertions following a
`reveal_type(something_of_type_never)` call can no longer be considered
unreachable.
