Relocate common setup - vars, flags, init()

Move any setup that is common to all tests - which includes setting global
variables, parsing CLI flags, and doing the pre-flight health check - to a
separate file called `common_setup_test.go` so that it's logically
separated.

The naming of this file seems to have no effect on the setup being performed
before running any of the tests. Presumably because it compiles all test
files that belong to the "main" package.
Move helper tests to helpers_test.go

So that they are logically separated from the actual helper code and match
the normal pattern of `foo_test.go` which tests `foo.go`.

I'm also removing the wrapper which called these individually - now that
they are in a `_test.go` file they will get picked up correctly themselves.
They can still be opted in with `-run Helper`. Plus we'll get the correct
errors if just one of them fail.
Merge pull request #22 from alphagov/relocate_setup_and_helper_tests

Relocate common setup - vars, flags, init()