Make 'DO NOT ADD STUPID TESTS' notice more prominent

Author: AlexWaygood

test_cases/README.md

@@ -4,19 +4,19 @@ This directory contains regression tests for the stubs found elsewhere in the
 typeshed repo. Each file contains a number of test cases, all of which should
 pass a type checker without error.
 
+**This directory should *only* contain tests for functions and classes which
+are known to have caused problems in the past, where the stubs are difficult to
+get right.** 100% test coverage for typeshed is neither necessary nor
+desirable, as it would lead to code duplication. Moreover, typeshed has
+multiple other mechanisms for spotting errors in the stubs.
+
 The internal structure of this directory should mimic typeshed as a whole.
 However, unlike the rest of typeshed, this directory largely contains `.py`
 files. This is because the purpose of this folder is to test the implications
 of typeshed changes for end users. For example, the stub for `builtins.pow` is
 found in `stdlib/builtins.pyi`, and the regression tests for `pow` are found in
 `test_cases/stdlib/test_builtins.py`.
 
-100% test coverage for typeshed is neither necessary nor desirable, as it would
-lead to unnecessary code duplication. Moreover, typeshed has multiple other
-mechanisms for spotting errors in the stubs. As such, this directory should
-only contain tests for functions and classes which are known to have caused
-problems in the past, where the stubs are difficult to get right.
-
 Note that, unlike elsewhere in typeshed, the test cases in this directory
 cannot always use modern syntax for type hints. For example, PEP 604 syntax
 (unions with a pipe `|` operator) is new in Python 3.10. While this syntax can