This PR splits up the nomad/mock package into more files. Specific features
that have a lot of mocks get their own file (e.g. acl, variables, csi, connect, etc.).
Above that, functions that return jobs/allocs/nodes are in the job/alloc/node file. And
lastly other mocks/helpers are in mock.go
This PR adds support for specifying checks in services registered to
the built-in nomad service provider.
Currently only HTTP and TCP checks are supported, though more types
could be added later.