Add parallelizable and randomly ordered tests support

[alvarhansen]

Resolves #335

Short description 📝

Adds parallelizable and randomly ordered tests support

Solution 📦

Adds parallelizable: Bool randomExecutionOrdering: Bool properties to TestableReference

Implementation 👩‍💻👨‍💻

Added tests to make sure default old schema files won't be affected by making sure default values are not written.

[yonaskolb]

Fantastic, thanks @alvarhansen! Could you check that the default alphabetical order is what Xcode uses when writing the scheme. Entries may potentially have to be added here https://github.com/tuist/xcodeproj/blob/master/Sources/xcodeproj/Extensions/AEXML%2BXcodeFormat.swift#L31

Could you also add a changelog entry

[alvarhansen]

@yonaskolb, turns out the ordering was not alphabetical. I added the order to XcodeFormat.swift.
Also updated the changelog.

[yonaskolb]

Awesome, great work!

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[pepicrft]

Thanks for adding this @alvarhansen 👏