Test values of buffers

[honno]

Mentioned in data-apis/dataframe-api#75 (comment) that we could test the values of buffers. Not sure how hard it'll be for all the types of examples we generate, so I'll at least give it a go to scope it out. My initial impression is our round-trip testing would end up covering the vast majority of problems anywho.

[honno]

My initial impression is our round-trip testing would end up covering the vast majority of problems anywho.

I think this is true. The round-trip tests cover testing the actual values of buffers, implicitly but extensively. Testing the values of buffers could be useful when trying to pinpoint issues, but the effort doesn't seem worth it.