Add test for --display-doctest-warnings option

src/test/rustdoc-ui/display-output.rs

+// check-pass
+// compile-flags:-Zunstable-options --display-doctest-warnings --test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+
+/// ```
+/// let x = 12;
+/// ```
+pub fn foo() {}

src/test/rustdoc-ui/display-output.stdout

+
+running 1 test
+test $DIR/display-output.rs - foo (line 6) ... ok
+
+successes:
+
+---- $DIR/display-output.rs - foo (line 6) stdout ----
+warning: unused variable: `x`
+  --> $DIR/display-output.rs:7:5
+   |
+LL | let x = 12;
+   |     ^ help: if this is intentional, prefix it with an underscore: `_x`
+   |
+   = note: `#[warn(unused_variables)]` on by default
+
+warning: 1 warning emitted
+
+
+
+successes:
+    $DIR/display-output.rs - foo (line 6)
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+