The supplied NuGet.config adds the EpiServer feed to your list
of feeds, so there should be little point in tinkering with feed
settings.
Right-click the Solution in the solution explorer and click "Restore NuGet Packages".
nuget restore
If you have fetched all dependencies using NuGet you can proceed to
build the solution by issuing xbuild.
TESTRUNNER=./packages/xunit.runner.console.2.1.0/tools/xunit.console.exe
mono $TESTRUNNER Tests/bin/Debug/Tests.dll