Update R# suggestion severity for 'ParameterTypeCanBeEnumerable'.
When a Convention class is present in the test assembly, stop assuming the DefaultConvention. When a TestAssembly class is present in the test assembly, stop assuming that all local Convention classes should be applied.
Convert SelfTestConvention from being a subclass of Convention to a static class that builds up an instance of Convention, so that it could be moved to the Fixie.Tests project without then being mistakenly found by the convention discovery code.
Move SelfTestConvention to Fixie.Tests, because it exists only for Fixie's tests-of-Fixie and would not be generally useful to other projects.