Method:
Select the Xcode project in the lefthand browser.
Click on your test target in the Project’s General tab.
Disclose “YorrPorojectNameTesting”. In my project the “Host Application” pulldown button was blank. Select your appropriate host application.
Try building your test target now. If that succeeds then converting Syntax should as well.
I had this problem after installing the Xcode 8 beta so I assume this is related.
http://stackoverflow.com/questions/37847807/xcode-8-beta-convert-to-current-swift-syntax-failed-could-not-find-test-host/37864933