Bump coverlet.msbuild from 6.0.3 to 6.0.4 in /src/Objectivity.AutoFixture.XUnit2.AutoFakeItEasy.Tests

Bump coverlet.msbuild from 6.0.3 to 6.0.4 in /src/Objectivity.AutoFixture.XUnit2.AutoFakeItEasy.Tests #450