cleanup: refactor "unit_test.go" Consul integration tests into proper e2e/unit tests

In https://github.com/hashicorp/nomad/pull/14546 we're removing a chunk of Consul integration testing out of the unit tests - these tests are _extremely_ invasive on implementation details of various components of Nomad's wrapper around Consul. Well written tests do not do this - instead functions should be tested individually, the whole integration should be covered in e2e testing.

Start by reviewing unit_test.go and the Consul integration tests it contains, then read the referenced pull request #14546 for the tests already being removed. Separate individual function coverage from whole-integration coverage, placing the former in unit tests and the latter in e2e tests; done means the invasive integration cases no longer remain in unit_test.go.

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