add describe.one: collection of tests with at least one passing

[arlimus]

This also includes:

• start the separation of expect and describe statements (i.e. give them their own file to be found more easily)
• bugfix: a global describe call without a block has resultet in a nil-checking failure

Next steps will have to target the example group structure, which is still bound to RSpec (last remaining close coupling) and the underlying check/test-registry.

[chris-rock]

Tested and works.

$ cat <<EOF >> test.rb
describe.one do
  describe file('/tmp') do
    it { should be_file }
  end
  describe file('/tmp') do
    it { should be_directory }
  end
end
EOF
$ inspec exec test.rb
.

Finished in 0.00059 seconds (files took 0.50709 seconds to load)
1 example, 0 failures

Crazy stuff. 🀄 Thanks @arlimus for bringing this to inspec.