Add test for get_rawio_class

[zm711]

After working on #1297 and @JuliaSprenger adding tests I thought it would be nice to mirror the same type of tests for the get_rawio_class. The two tests I've added are:

1. First test is for a supported file format which should return a reader
2. The second is for a non-supported file format which should return None

One question I wanted to bring up is that currently neo.io has a function get_io whereas neo.rawio instead has a get_rawio_class instead. It might be more parsimonious to change to get_rawio instead, but rather than just make that change I wanted to ask for opinions first. Happy to add here or raise an issue if that is preferred.

[apdavison]

thanks for this.

Renaming to get_rawio seems like a good idea for consistency. I suggest we add that as an alias, and emit a deprecation warning for the original name, get_rawio_class. Please add an issue for this, and any PR would be welcome.