Fix tests not building correctly in BoxContentSDKTests target.

[irvingruan]

• Comment out BOXAPIMultipartPiece-related tests since it was removed in new NSURLSession changes.

[boxcla]

Hi @irvingruan, thanks for the pull request. Before we can merge it, we need you to sign our Contributor License Agreement. You can do so electronically here: http://opensource.box.com/cla

Once you have signed, just add a comment to this pull request saying, "CLA signed". Thanks!

[CLAassistant]

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[stale[bot]]

This issue has been automatically marked as stale because it has not been updated in the last 30 days. It will be closed if no further activity occurs within the next 7 days. Feel free to reach out or mention Box SDK team member for further help and resources if they are needed.

[stale[bot]]

This issue has been automatically closed due to maximum period of being stale. Thank you for your contribution to Box iOS SDK and feel free to open another PR/issue at any time.