Added TestContext.Write Is Obsolete Analyzer

[manfred-brands]

Fixes #770

[mikkelbu]

LGTM. Only some minor nit-picks.

I've not had time to follow the discussion in the NUnit repo, but as far as I can tell these methods have not been marked as obsolete yet in the framework, so this Analyzer might confuse users by breaking the compilation - severity error - although the framework is fine, so perhaps this should just be warning (or perhaps we should rephrase the text a little to make it explicit, that these methods will be obsolete and then removed in future versions of the framework?)

[mikkelbu]

Just noticed that we have forgotten to update the sln file with the new markdown file

[manfred-brands]

I've not had time to follow the discussion in the NUnit repo, but as far as I can tell these methods have not been marked as obsolete yet in the framework, so this Analyzer might confuse users by breaking the compilation - severity error - although the framework is fine, so perhaps this should just be warning (or perhaps we should rephrase the text a little to make it explicit, that these methods will be obsolete and then removed in future versions of the framework?)

I have both change the level to Warning and rephrased it from are Obsolete into will be marked as Obsolete and eventually removed