Add tests for new doc alias check

Author: GuillaumeGomez

src/test/rustdoc-ui/doc-alias-same-name.rs

@@ -0,0 +1,4 @@
+#![crate_type = "lib"]
+
+#[doc(alias = "Foo")] //~ ERROR
+pub struct Foo;

src/test/rustdoc-ui/doc-alias-same-name.stderr

@@ -0,0 +1,8 @@
+error: `#[doc(alias = "...")]` is the same as the item's name
+  --> $DIR/doc-alias-same-name.rs:3:7
+   |
+LL | #[doc(alias = "Foo")]
+   |       ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/doc-alias-same-name.rs

@@ -0,0 +1,4 @@
+#![crate_type = "lib"]
+
+#[doc(alias = "Foo")] //~ ERROR
+pub struct Foo;

src/test/ui/doc-alias-same-name.stderr

@@ -0,0 +1,8 @@
+error: `#[doc(alias = "...")]` is the same as the item's name
+  --> $DIR/doc-alias-same-name.rs:3:7
+   |
+LL | #[doc(alias = "Foo")]
+   |       ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+