Move Rake tasks for testing to :test namespace

The result:

    rake test:all            # Run tests for all
    rake test:clobber_rcov   # Remove rcov products for rcov
    rake test:functional     # Run tests for functional
    rake test:rcov           # Analyze code coverage with tests
    rake test:unit           # Run tests for unit

The `test` task is an alias for `test:all` and also happens to
be the default when no task is given.