backend/local: refactor tests with modern state and default providers

[mildwonkey]

I copied a couple of helper functions from terraform: there was little enough to copy and we don't expect to be writing tons of new backends so that seemed good enough for this purpose.

[mildwonkey]

proof that backend tests are happy:

go test ./backend/...
ok  	github.com/hashicorp/terraform/backend	(cached)
ok  	github.com/hashicorp/terraform/backend/atlas	(cached)
ok  	github.com/hashicorp/terraform/backend/init	(cached)
ok  	github.com/hashicorp/terraform/backend/local	(cached)
ok  	github.com/hashicorp/terraform/backend/remote	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/artifactory	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/azure	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/consul	1.058s
ok  	github.com/hashicorp/terraform/backend/remote-state/cos	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/etcdv2	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/etcdv3	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/gcs	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/http	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/inmem	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/manta	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/oss	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/pg	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/s3	(cached)
ok  	github.com/hashicorp/terraform/backend/remote-state/swift	(cached)

[ghost]

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