Remove unused TestCase subclass

We were going to use this to implement RDoc::TestCase features in the
MiniTest::Spec::DSL, however TestCase won't work this way and will have
to reconsider another way.
Move spec/sdoc/* to spec/*

Since we only have a one spec file right now, I'm just going to keep
them all under ./spec
Update Rake::TestTask to reflect re-organization of spec folder
Use RDoc::TopLevel#last_modified instead of #file_stat.mtime