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

[Pointerbender]

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:
- rust-lang/unsafe-code-guidelines#258
- rust-lang/unsafe-code-guidelines#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.

[RalfJung]

Looking good, thanks. :-)
@bors r+

[bors]

📌 Commit 386863a has been approved by RalfJung

[bors]

⌛ Testing commit 386863a with merge 44122f9...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 44122f9 to master...