Auto merge of #1824 - Pointerbender:strings-test, r=RalfJung

added a strings.rs regression test case for potential future UB

This PR adds a regression test for the aliasing rules of a `Unique<T>` pointer.
    At the time of writing this test case, Miri does not treat `Unique<T>`
    pointers as a special case, these are treated like any other raw pointer.
    However, there are existing Github issues which may lead to `Unique<T>`
    becoming a special case through asserting unique ownership over the pointee:
    - https://github.com/rust-lang/unsafe-code-guidelines/issues/258
    - https://github.com/rust-lang/unsafe-code-guidelines/issues/262
    In the new test case, the calls to `String::remove` and `String::insert[_str]` follow
    code paths that would trigger undefined behavior in case `Unique<T>`
    would ever assert semantic ownership over the pointee. Internally,
    these methods call `self.vec.as_ptr()` and `self.vec.as_mut_ptr()` on
    the vector of bytes that are backing the `String`. That `Vec<u8>` holds a
    `Unique<u8>` internally. The second call to `Vec::as_mut_ptr(&mut self)`
    would then invalidate the pointers derived from `Vec::as_ptr(&self)`.
    Note that as long as `Unique<T>` is treated like any other raw pointer,
    this test case should pass. It is merely here as a canary test for
    potential future undefined behavior.