docs(code-style-guide): Add unit test function name rule

modules/contributor/pages/code-style-guide.adoc

@@ -296,7 +296,7 @@ The usage of `thiserror` is considered invalid.
 ----
 #[derive(Snafu)]
 enum Error {
-    #[snafu(display("failed to read config file of user {user_name}"))]
+    #[snafu(display("failed to read config file of user {user_name:?}"))]
     FileRead {
         source: std::io::Error,
         user_name: String,
@@ -595,3 +595,63 @@ let memory: CpuQuantity = "2".parse();
 ----
 
 ====
+
+== Writing tests
+
+=== Unit test function names
+
+Function names of unit tests must not include a redundant `test` prefix or
+suffix.
+
+It results in the output of `cargo test` containing superfluous mentions of
+"test", especially when the containing module is called `test`. For example:
+`my_crate::test::test_valid`.
+
+Instead, use an appropriate name to describe what is being tested. The previous
+example could then become: `my_crate::test::parse_valid_api_version`.
+
+[TIP.code-rule,caption=Examples of correct code for this rule]
+====
+
+[source,rust]
+----
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn parse_valid_api_version() {
+        todo!()
+    }
+
+    #[test]
+    fn parse_invalid_api_version() {
+        todo!()
+    }
+}
+----
+
+====
+
+[WARNING.code-rule,caption=Examples of incorrect code for this rule]
+====
+
+[source,rust]
+----
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_valid() {
+        todo!()
+    }
+
+    #[test]
+    fn test_invalid() {
+        todo!()
+    }
+}
+----
+
+====