Add UI regressions tests for rustdoc --show-coverage option

tests/rustdoc-ui/show-coverage-json.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: -Z unstable-options --show-coverage --output-format=json
+//@ check-pass
+
+mod bar {
+    /// a
+    ///
+    /// ```
+    /// let x = 0;
+    /// ```
+    pub struct Foo;
+}
+
+pub use bar::Foo;

tests/rustdoc-ui/show-coverage-json.stdout

@@ -0,0 +1 @@
+{"$DIR/show-coverage-json.rs":{"total":2,"with_docs":1,"total_examples":2,"with_examples":1}}

tests/rustdoc-ui/show-coverage.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: -Z unstable-options --show-coverage
+//@ check-pass
+
+mod bar {
+    /// a
+    ///
+    /// ```
+    /// let x = 0;
+    /// ```
+    pub struct Foo;
+}
+
+pub use bar::Foo;

tests/rustdoc-ui/show-coverage.stdout

@@ -0,0 +1,7 @@
++-------------------------------------+------------+------------+------------+------------+
+| File                                | Documented | Percentage |   Examples | Percentage |
++-------------------------------------+------------+------------+------------+------------+
+| ...ests/rustdoc-ui/show-coverage.rs |          1 |      50.0% |          1 |      50.0% |
++-------------------------------------+------------+------------+------------+------------+
+| Total                               |          1 |      50.0% |          1 |      50.0% |
++-------------------------------------+------------+------------+------------+------------+