tests: fix state version download test

[ctrombley]

Description

This PR updates the state version download test to poll the resource and ensure the download URL is available before proceeding.

External links

• JIRA

[brandonc]

Lgtm

[datadog-terraform-cloud-hashicorp]

⚠️ Tests

⚠️ Warnings

❄️ 1 New flaky test detected

TestWorkspaces_AddTags/successfully_adds_tags_by_id_and_name from workspace_integration_test.go (Datadog)

	Error Trace:	/home/runner/work/go-tfe/go-tfe/workspace_integration_test.go:2950
	            				/home/runner/go/pkg/mod/github.com/!data!dog/dd-trace-go/v2@v2.5.0/internal/civisibility/integrations/gotesting/instrumentation_orchestrion.go:289
	            				/home/runner/go/pkg/mod/github.com/!data!dog/dd-trace-go/v2@v2.5.0/internal/civisibility/integrations/gotesting/instrumentation_orchestrion.go:293
	Error:      	Should NOT be empty, but was <nil>
	Test:       	TestWorkspaces_AddTags/successfully_adds_tags_by_id_and_name

ℹ️ Info

🧪 All tests passed

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[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.