Improve regression test documentation (#10935)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

test_cases/README.md

@@ -32,6 +32,13 @@ stubs where decisions have been taken that might be slightly unusual. These
 test cases serve a different purpose: to check that type checkers do not emit
 false-positive errors for idiomatic usage of these classes.
 
+## Running the tests
+
+To verify the test cases in this directory pass with mypy, run `python tests/regr_test.py stdlib`
+from the root of the typeshed repository. This assumes that the development
+environment has been set up as described in the [CONTRIBUTING.md](../CONTRIBUTING.md)
+document.
+
 ### How the tests work
 
 The code in this directory is not intended to be directly executed. Instead,

tests/README.md

@@ -104,7 +104,8 @@ the stubs in typeshed (including the standard library).
 ## regr\_test.py
 
 This test runs mypy against the test cases for typeshed's stdlib and third-party
-stubs. See the README in the `test_cases` directory for more information about what
+stubs. See [the README in the `test_cases` directory](../test_cases/README.md)
+for more information about what
 these test cases are for and how they work. Run `python tests/regr_test.py --help`
 for information on the various configuration options.
 

tests/regr_test.py

@@ -69,7 +69,10 @@ parser.add_argument(
     type=package_with_test_cases,
     nargs="*",
     action="extend",
-    help="Test only these packages (defaults to all typeshed stubs that have test cases)",
+    help=(
+        "Test only these packages (defaults to all typeshed stubs that have test cases). "
+        'Use "stdlib" to test the standard library test cases.'
+    ),
 )
 parser.add_argument(
     "--all",