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.