tests: Change dd argument to use a bytes value

[algochoi]

Summary

Changes dd's bs argument to a bytes value to work on both macOS (specifically, CircleCI's macOS 11) and linux. Also added a comment above the line to justify this:

# Some of our MacOS nightly tests fail for specifying the bs (block size)
# value in capital letters (i.e. 11M), so just specify it as 1024 bytes and
# allocate 11K blocks so we get a 11MB sized file. 

Nightly integration tests seem to pass now: https://app.circleci.com/pipelines/github/algorand/go-algorand/13522/workflows/2cca47be-d835-48c3-897e-7f713e521534

[ahangsu]

lookin good