Add regression tests for suspicious_doc_comments

Author: samueltardieu

tests/ui/suspicious_doc_comments.fixed

@@ -87,4 +87,8 @@ pub mod useless_outer_doc {
     use std::mem;
 }
 
+// Do not lint, this is not a `///!`
+#[doc = "! here's some docs !"]
+fn issue14265() {}
+
 fn main() {}

tests/ui/suspicious_doc_comments.rs

@@ -87,4 +87,8 @@ pub mod useless_outer_doc {
     use std::mem;
 }
 
+// Do not lint, this is not a `///!`
+#[doc = "! here's some docs !"]
+fn issue14265() {}
+
 fn main() {}