tests: Add note about uniqueness requirement for test suite names

Author: practicalswift

doc/developer-notes.md

@@ -72,7 +72,8 @@ code.
   - Class names, function names and method names are UpperCamelCase
     (PascalCase). Do not prefix class names with `C`.
   - Test suite naming convention: The Boost test suite in file
-    `src/test/foo_tests.cpp` should be named `foo_tests`.
+    `src/test/foo_tests.cpp` should be named `foo_tests`. Test suite names
+    must be unique.
 
 - **Miscellaneous**
   - `++i` is preferred over `i++`.