tests: adjust some augmented-assignment* tests

- `tests/ui/augmented-assignment-feature-gate-cross.rs`:
  - This was *originally* to feature-gate overloaded OpAssign
    cross-crate, but now let's keep it as a smoke test.
  - Renamed as `augmented-assignment-cross-crate.rs`.
  - Relocated under `tests/ui/binop/`.
-  `tests/ui/augmented-assignments.rs`:
  - Documented test intent.
  - Moved under `tests/ui/borrowck/`.
- `tests/ui/augmented-assignment-rpass.rs`:
  - Renamed to drop the `-rpass` suffix, since this was leftover from
    when `run-pass` test suite was a thing.
  - Moved under `tests/ui/binop/`.

tests/ui/binop/augmented-assignments-cross-crate.rs

@@ -1,3 +1,5 @@
+//! Smoke test for overloaded compound assignments cross-crate.
+
 //@ run-pass
 //@ aux-build:augmented_assignments.rs
 

tests/ui/borrowck/augmented-assignments.rs

@@ -1,3 +1,6 @@
+//! Check that overloaded compound assignment operators respect usual borrowck rules and emit
+//! reasonable diagnostics.
+
 use std::ops::AddAssign;
 
 #[derive(Clone)]

tests/ui/borrowck/augmented-assignments.stderr

@@ -1,5 +1,5 @@
 error[E0505]: cannot move out of `x` because it is borrowed
-  --> $DIR/augmented-assignments.rs:17:5
+  --> $DIR/augmented-assignments.rs:20:5
    |
 LL |     let mut x = Int(1);
    |         ----- binding `x` declared here
@@ -10,7 +10,7 @@ LL |     x;
    |     ^ move out of `x` occurs here
 
 error[E0596]: cannot borrow `y` as mutable, as it is not declared as mutable
-  --> $DIR/augmented-assignments.rs:24:5
+  --> $DIR/augmented-assignments.rs:27:5
    |
 LL |     y
    |     ^ cannot borrow as mutable