sdcard: Adding tests for csd parsing.

[benwynn]

On my last PR that updated the parsing of the CSD register, I was asked if a test could be provided.

Here's a test for the csd parsing, with csd values recorded from the two cards I currently have available.

I've ordered some smaller cards so I can record and test additional values, but happy to have others contribute test-cases as well.

[benwynn]

@projectgus Here's a test of the bits I changed recently, at least.