Add test to ensure nat_rules order is preserved
Remove magic number for NatService Id
Remove test_cases array, use regular tests instead.

The test_cases iteration was a trial, and its usefulness
was very limited. Update tests so that they are individual
it 'blah' do .. end blocks
Replace use of constand for BASE_NAT_ID

... with a regular rspec instance variable.

The use of the constant was a hack required by the test_cases
array, refactored out in the previous commit.
Merge pull request #10 from alphagov/nat_rule_order

[#64582784] Add test to ensure nat_rules order is preserved