When I get a test failure, which triggers copying of screenshots, I get this error in my log:
Exporting attachments
Failed to export UI test artifacts, error no test attachments found at: /Users/vagrant/Library/Developer/Xcode/DerivedData/LogicPuzzles-gewhzekhsopbesfofqckztpeajlv/Logs/Test/Attachments
xcode test exit code: 65
It seems that Xcode 10 has moved the location of the generated screenshots – probably to support multiple simultaneous simulators.