I moved pipenv installing from "init" section to "install" and the Python version has become 3.5.6. But tests still fall for no apparent reason. It seems that the first test in which GUI is created falls.
It turned out that the initial problem was caused by the crash of a third-party library. Without xvfb-run wxPython does not work at server and all GUI tests fall. I run all the tests locally without xvfb-run.