Add tests for ReadAt of ByteView

[sum12]

The tests pass if ReadAt returns in errors for invalid values of offset.
The patch also adds check for error to be returned when dest is larger
in size than source.

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[sum12]

@googlebot I signed it

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

CLAs look good, thanks!

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[googlebot]

CLAs look good, thanks!

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[sum12]

@bradfitz could you please have a look at this ?
does this help in anyway ?