Add AST unpretty test for unsafe attribute

Author: Urgau

tests/ui/unpretty/unsafe-attr.rs

@@ -0,0 +1,11 @@
+//@ compile-flags: -Zunpretty=normal
+//@ check-pass
+
+#[no_mangle]
+extern "C" fn foo() {}
+
+#[unsafe(no_mangle)]
+extern "C" fn bar() {}
+
+#[cfg_attr(FALSE, unsafe(no_mangle))]
+extern "C" fn zoo() {}

tests/ui/unpretty/unsafe-attr.stdout

@@ -0,0 +1,11 @@
+//@ compile-flags: -Zunpretty=normal
+//@ check-pass
+
+#[no_mangle]
+extern "C" fn foo() {}
+
+#[no_mangle]
+extern "C" fn bar() {}
+
+#[cfg_attr(FALSE, unsafe(no_mangle))]
+extern "C" fn zoo() {}