Add tests for `instanceof` behavior

Add tests for `instanceof` behavior

In preparation for possible future refactoring for these classes to return a `Proxy` object (this pattern is required to simulate dynamic superclasses, e.g., `CSVReader` returning either `ReadableStream` OR `TransformStream` depending on constructor argument). If such refactoring takes place, these tests will check that `Proxy` was implemented correctly and maintain backwards compatibility.