tests/target: Add failing test for rustdoc in enum

Test case as reported in rust-lang#6280.
This test case used to fail, but the code in the previous commit fixes
it.

We followed instructions on Contributing.md [1] to create a test case.

`tests/target/rust-doc-in-enum/vertical-no-doc.rs` is being left unformatted
(expected behavior), while `tests/target/rust-doc-in-enum/vertical-with-doc.rs`
is formatted to `C { a: usize }` (unexpected behavior).

The only different between the two samples is the `/// C` rustdoc added
in `with-doc.rs`.
This reproducing example is minimal: for example, after removing `d: usize`
we do not reproduce the reported behavior.

We found this issue while adding rustdocs to an existing project.
We would expect that adding a line of documentation should not cause the
formatting of the code to change.

[1] https://github.com/rust-lang/rustfmt/blob/40f507526993651ad3b92eda89d5b1cebd0ed374/Contributing.md#L33

Author: malikolivier

tests/target/rust-doc-in-enum/vertical-no-doc.rs

@@ -0,0 +1,12 @@
+enum A {
+    B {
+        a: usize,
+        b: usize,
+        c: usize,
+        d: usize,
+    },
+
+    C {
+        a: usize,
+    },
+}

tests/target/rust-doc-in-enum/vertical-with-doc.rs

@@ -0,0 +1,13 @@
+enum A {
+    B {
+        a: usize,
+        b: usize,
+        c: usize,
+        d: usize,
+    },
+
+    /// C
+    C {
+        a: usize,
+    },
+}