chore: Improve copilot instructions for tests (#225)

Author: borchero

.github/instructions/tests.instructions.md

@@ -0,0 +1,9 @@
+---
+applyTo: tests/**/*.py
+---
+
+# Testing Guidelines
+
+- Instead of writing new tests, existing tests should ideally be parametrized using `@pytest.mark.parametrize`
+  unless the parametrization is impractical, e.g. by adding more parameters
+- Tests should not use docstrings unless they are _very_ complex