Fix running tests

[wiruzx]

It seems that after enabling Library Evolution Support in #677 not all test cases are running.

There is one test case in Chatto - ChatItemCompanionCollectionTests. And a few of them in ChattoAdditions.

If XCTestCase uses a struct from Chatto it won't run. Importing it as @testable doesn't seem to make any difference.

Removing BUILD_LIBRARY_FOR_DISTRIBUTION option solves the issue. Making the struct @frozen also solves the issue.

If you run the test case individually, it'll work.

My environment:

• Xcode 12 & Xcode 11.7
• macOS 10.15.6