Replace const.txt file input with synthetic inputs

This commit replaces file resource-based test inputs with in-memory
equivalents.  This is more consistent and efficient than the previous
approach.  Also resized some test inputs to be partSize + 1 instead of
2 * partSize.  Tested against aws-s3, blobstore, core, cloudfiles-us,
and filesystem.
[minor] `protected`, just to stay consistent with the others?