This PR adds a set of tests to the Consul test suite for testing
Nomad OSS's behavior around setting Consul Namespace on groups,
which is to ignore the setting (as Consul Namespaces are currently
an Enterprise feature).
Tests are generally a reduced facsimile of existing tests, modified
to check behavior of when group.consul.namespace is set and not set.
Verification is oriented around what happens in Consul; the in-depth
functional correctness of these features is left to the original tests.
Nomad ENT will get its own version of these tests in `namespaces_ent.go`.
