Add support for snapshot testing

[jguionnet]

Describe the solution you'd like
helm-unittest as a great feature called snapshot testing.

Sometimes you may just want to check that the rendered manifest did not change between helm package version without every details asserted.

Describe alternatives you've considered
None and neither did the community, see https://github.com/orgs/gruntwork-io/discussions/780

Additional context

[Mo0rBy]

Hello, I just came across this issue simply because it was fresh and at the top of the page.
For context, please read this article. I believe this is how Camunda test their own Helm charts. It describes "Golden template files" and I think these are essentially snapshots.

Could be what you are looking for, or could be used for inspiration to add snapshots to this already great tool.

[jguionnet]

@Mo0rBy Thanks for the reference. The linked pull request (#1378) implements it for helm install, which does not have the TS and other generated ID issues. I feel it is better than the implementation from Camunda as it provides a diff git like which show all the difference, see the gist referenced in the PR