I was suffering from this problem for long times.
I had about 100 projects different version was deployed in different server.
Updating xunit from 2.2.0 to 2.3.1 was not a solution because build was failing in 2.3.1.
Then I just updated xunit.runner.visualstudio to 2.3.1 and everything started to work fine. I have used this command in my package manager console to updated my xunit.runner.visualstudio package
Get-Project ComapanyName.ProjectName.*.Tests | Install-Package xunit.runner.visualstudio -Version 2.3.1