Partial reads were performed once using a partial size of 1 byte.
This was not enough to detect problems which only occur in combination
with IOExceptions. Partial reads are now only applied when no exception
is thrown.

Moreover, the tests didn't check whether the total number of sampled bytes
is what it is supposed to be. Added a field to the data dumps checking
the total number of bytes in the sampled data.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181296545