I had the same problem, and found a workaround. This appeared in the build log:
note: did not find header 'XCTest.h' in framework 'XCTest' (loaded from '/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.0.sdk/Developer/Library/Frameworks')
Sure enough, the XCTest.framework in that path was missing a Headers directory. Either they forgot to copy it in, or they're linking against the incorrect framework.
I was able to fix the problem by adding a symbolic link to the current Xcode's XCTest's Headers directory:
cd /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.0.sdk/Developer/Library/Frameworks/XCTest.framework ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/Library/Frameworks/XCTest.framework/Headers
(If your Xcode installations aren't in /Applications, don't forget to change the top of those paths appropriately for your installation.)
Once I did that, then my project built correctly.