[#71835572] Convert tests to use vCloud Tools Tester

Convert integration tests to use the [vCloud Tools Tester](rubygems.org/gems/vcloud-tools-tester) gem, which replaces the use of environment variables to define test parameters.

Adds a new development dependency for 'vcloud-tools-tester' and a  vcloud_tools_testing_config.yaml.template` file. Also, update the README and .gitignore file accordingly.

I'd like to refactor the `@test_data` variables to use `let` rather than put them in the `before` block but think this should be in a separate PR.

Much of this is cribbed from @mikepea's https://github.com/alphagov/vcloud-core/pull/55.

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Also, some housekeeping and DRY up some of the tests.