Improve error handling in test modules: test_extensible, test_bitmapset

Hello

The recently added test extensible module has a small oversight: it
lacks input validation, which made it easy to crash the backend with
invalid input with the test module loaded. While this doesn't seem
like a critical issue for a test module, the commit message also
mentions that this could be a template for extension developers, so I
think it is worth fixing.

There's a similar issue in test_bitmapset: that's an older commit, but
it shows the same issue.

I attached fixes to both as 0001/0002.