Add docs for tests modules: common, common::assert, common::test.

This is to better explain their purpose.

src/routes/mod.rs

@@ -75,6 +75,9 @@ pub mod tests {
     use rstest::rstest;
     use tempfile::TempDir;
 
+    /// Contains reusable code which can be shared across different tests, like response assertions,
+    /// fixtures to set up the test environment and particular test implementations
+    /// which can be seeded with different values, but the implementation scheme remains the same.
     pub mod common {
         use crate::app::configure_rocket;
         use ::rocket::http::ContentType;
@@ -256,6 +259,7 @@ pub mod tests {
             "token-pseudo-random"
         }
 
+        /// Contains reusable assertion helpers.
         pub mod assert {
             use ::rocket::http::{ContentType, Status};
             use ::rocket::local::blocking::LocalResponse;
@@ -312,6 +316,7 @@ pub mod tests {
             }
         }
 
+        /// Contains reusable test implementations.
         pub mod test {
             use super::*;
             use ::rocket::http::{ContentType, Status};