add convention about testing

Signed-off-by: Aleksandr Motsjonov <soswow@gmail.com>

Author: soswow

src/python-nanobind/MIGRATION_STATUS.md

@@ -40,3 +40,5 @@ These exist only under `src/python/` today; there are **no** corresponding `py_*
 ## Conventions
 
 When adding coverage, prefer mirroring the existing `declare_*` split in `src/python/` unless a file becomes too large.
+
+Extend **testsuite** coverage for any migrated code that is not already covered, so **parity with pybind11** is demonstrated rather than only claimed. Follow the existing `testsuite/python-*` scripts and `ref/out.txt` pattern where applicable.