Clean up tinkerbell E2E test set up

**What would you like to be added**: Clean up tinkerbell E2E test set up

**Why is this needed**:

Currently, there are redundancies on how to bring up / tear down clusters on tinkerbell across different tests. Examples:
- https://github.com/aws/eks-anywhere/blob/5496d7716cc63c06e67e99048c4615bde8317caa/test/e2e/curated_packages_test.go#L235
- https://github.com/aws/eks-anywhere/blob/82942b8cfbebb33fb9a00bcda9b048d845904ef8/test/e2e/emissary_test.go#L48

Te better follow the DRY principle, we should move them out to a more generic function.

Start with test/e2e/curated_packages_test.go around line 235 and test/e2e/emissary_test.go around line 48. Compare how each test brings up and tears down Tinkerbell clusters, then identify the shared setup and cleanup flow. Done means the redundant flows use a generic function and the affected E2E tests still run successfully.

Written by the indexing model from the issue text.