Fix deadlock running tests in Xcode 16

[stephan-tolksdorf]

Fixes #56 by checking in the load initializer whether an XCTest environment variable is set and if so, dispatch the initialization asynchronously on the main thread. Also provides a way for disabling the load time initialization with the environment variable STULabel_NoMainScreenPropertiesInitializationOnLoad and then explicitly performing the initialization with the stu_initializeMainScreenProperties function.

[macdrevx]

I'd love for there to be an option like STULabel_NoMainScreenPropertiesInitializationOnLoad but that we can use with the shipping app.

[macdrevx]

Maybe a custom Info.plist key? Are those save to read during +load?

[macdrevx]

Would be better if it were something that was determined at compile time though.