How to build test using mono

I have to write tests for a project done by some other peoples. I think they are using .NET Framework version 4.5.2 and using mono to build the project on a Linux system. Now on the same system, I wrote some tests using XUnit and .NET core. But when I run these tests using dotnet test I get the error that the .NET framework is not installed(as the reference projects are on this). But I can't install the .NET Framework on Linux. How to run these tests on the Linux system?()