Pull out firewall tests from general VSE tests

There is too much complexity in the general VSE test, so this pulls the
firewall specific testing out into its own file.

This is largely to allow us to get quicker feedback on specific tests with
the VSE.
Bring all firewall tests under same context

Particularly with the functions, we need to take care to ensure that
we are defining everything in our own scope/context - otherwise redefines
will overwrite and thing break.

Also changed names of context and some test names to improve readability.
Clarify run order of firewallService test

sub-contexts are run after any declarations at the
higher level, but are useful for documentation.

Make all tests at the same context layer, which ensures
that they run in a top-to-bottom order
Split NatService specific checks into own test

As per the FirewallService checks in previous commits.
Adjust contexts within NatService test

So that we are clear in the run order of the tests.

Also push the function declarations under the NatService specific
context scope, so as to not conflict with FirewallService test
Remove NatService and FirewallService specific tests

... leaving us with a clean slate for the 'play nicely with
all services' tests.
Fix up scope/context for general VSE tests
Clarify purpose of update_configuration test

The call to Core::EdgeGateway.update_configuration supports updating
single or multiple services at one time -- and is expensive.

We should ensure that we only ever call it once for a given configuration update.
Ensure that we actually update FirewallService

Specifically:

* Test that we are starting from a clean slate
* Test that we create some rules (effectively, test that our generator works)
* Test that the number of remote rules matches the local number.
DRY out FirewallService tests
improve test title
Remove unneccessary helper method
Remove redundant test

This is now tested more cleanly in the update context
Merge branch 'master' into improve-vse-tests
Add basic tests for Nat+Firewall

so that we can be sure that updates are fine with multiple services

NB: to be expanded in other stories, basic tests for now
Merge pull request #139 from alphagov/improve-vse-tests

[#62380500] Split out Firewall, Nat, and General VSE integration tests