Add test ensuring that drop terminator of no-drop type is a NOP

[RalfJung]

In rust-lang/rust#103957 we realized that Miri's treatment of Drop terminators subtly differed from that of the codegen backends. That got fixed, but we don't have a test for that. Not sure if a test is even possible, but we custom_mir it might be?

Cc @JakobDegen

[JakobDegen]

It will be once I add storage statements at least. Will do so then