Remove nunit.analyzers.integrationtests and nunit.analyzers.playground projects

[mikkelbu]

Currently, we don't use the nunit.analyzers.integrationtests and nunit.analyzers.playground projects, so I think we should remove them (nunit.analyzers.integrationtests are not even part of the solution).

The only things that perhaps should be moved from integrationtests are tests of functionality when we use "using static directive" (using static NUnit.Framework.Assert;) or aliasing (using NF = NUnit.Framework;). Also integrationtests have tests of optional parameters and params that we might lack in the tests project.