test: Add two tests for issue rust-lang#6272.

src/test/run-fail/borrowck-wg-autoderef-and-autoborrowvec-combined-fail-issue-6272.rs

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+// error-pattern:borrowed
+
+// Issue #6272. Tests that freezing correctly accounts for all the
+// implicit derefs that can occur and freezes the innermost box. See
+// the companion test
+//
+//     run-pass/borrowck-wg-autoderef-and-autoborrowvec-combined-issue-6272.rs
+//
+// for a detailed explanation of what is going on here.
+
+fn main() {
+    let a = @mut [3i];
+    let b = @mut [a];
+    let c = @mut b;
+
+    // this should freeze `a` only
+    let x: &mut [int] = c[0];
+
+    // hence this should fail
+    a[0] = a[0];
+}

src/test/run-pass/borrowck-wg-autoderef-and-autoborrowvec-combined-issue-6272.rs

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+// Issue #6272. Tests that freezing correctly accounts for all the
+// implicit derefs that can occur.
+//
+// In this particular case, the expression:
+//
+//    let x: &mut [int] = c[0];
+//
+// is seen by borrowck as this sequence of derefs
+// and pointer offsets:
+//
+//    &*((**c)[0])
+//
+// or, written using `x.*` for `*x` (so that everything
+// is a postfix operation):
+//
+//    &c.*.*.[0].*
+//       ^    ^
+//       |    |
+//       b    a
+//
+// Here I also indicated where the evaluation yields the boxes `a` and
+// `b`. It is important then that we only freeze the innermost box
+// (`a`), and not the other ones (`b`, `c`).
+//
+// Also see the companion test:
+//
+// run-fail/borrowck-wg-autoderef-and-autoborrowvec-combined-fail-issue-6272.rs
+
+
+fn main() {
+    let a = @mut [3i];
+    let b = @mut [a];
+    let c = @mut b;
+
+    // this should freeze `a` only
+    let _x: &mut [int] = c[0];
+
+    // hence these writes should not fail:
+    b[0] = b[0];
+    c[0] = c[0];
+}