Not overwrite configurations and defaultOptions keys

[Ernest0-Production]

When modifying xctestplan, information in the fields "configurations", "defaultOptions" is erased.

[mikeger]

Hey, good point. For the moment, the test plan is parsed as Codable, so all the new keys added by Xcode team would be ignored. This approach needs to be re-visited to make the tool future-proof.

[mikeger]

@Ernest0-Production this PR should address the issue: #83