Add a test for buffers that cannot hold complete values

[zslayton]

We have a test that handles reading values across 'seam's in the default 4k buffer size. We should add a more specific/explicit test that uses a custom buffer size.

Under the covers we create a bufio.Reader with a default (4096-byte) internal buffer. After reading 4 bytes of BVM and 511 x 8 byte integer values, there are 4 bytes remaining in that buffer, plus another 4 in the underlying stream. Pre-fix, we’d do a partial read of those four bytes as a (corrupt) 512th integer, then try to read the remaining four bytes as another value.

(Might be worth trying to add a more specific test with a custom Reader so it’s more obvious what’s going on, if the bufio.Reader doesn’t get in the way? This is the code I wrote to debug the issue and figured I might as well include :))

Originally posted by @fernomac in ca20906