Analyzer and fix: make test method static.

[JohanLarsson]

Probably default disbled making it opt-in. Nice convenience for refactoring if nothing else.

[mikkelbu]

Sounds like a nice addition, albeit I usually have instance test methods (too lazy to write the static :)).

[pantosha]

@JohanLarsson FxCop analyzers already has rule CA1822 with similar behavior.