Hi,
There are patches on swift-package-manager to get it to bootstrap now with CMake 3.15. There are a couple of patches which still need to be re-visited to actually get them merged (the patches are on GitHub).
As to your point of the tests - I think that you have something not setup or built improperly. The full test suite for the compiler, standard library should pass. The test suite for libdispatch should pass. The test suite for Foundation still have some failures due to implementation items which still needs to be addressed. The XCTest test suite passes except for 2 failures which are known and diagnosed (there is a SR tracking to address the underlying issue).