Feat/add snapshot based testing for helm

[jguionnet]

Description

Fixes #1377

Add a set of helper functions, like the UnmarshalK8SYaml functions.

• UpdateSnapshot creates/refreshes a local snapshot
• DiffAgainstSnapshot diffs the manifest generated by the test against the local snapshot. It returns the number of differences that can be asserted in a test. It also displays the difference between the two sets of k8s manifests in git style.

See test output in this gist https://gist.github.com/jguionnet/376e768009a244d3315f31b82598aa78

TODOs

Read the Gruntwork contribution guidelines.

• [ x] Update the docs. (ie provided sample tests as documentation)
• [ x] Run the relevant tests successfully, including pre-commit checks.
• [ x ] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [ x ] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Added support in Helm for snapshot testing, similar to helm-unittest feature snapshot testing.

Migration Guide

NA

[denis256]

This branch has conflicts that must be resolved
Use the [web editor](https://github.com/gruntwork-io/terratest/pull/1378/conflicts) or the  to resolve conflicts.
Conflicting files
go.mod

[jguionnet]

@denis256: I resolved the conflict, could you please review it?

[jguionnet]

@denis256. Thanks for reviewing. I fixed it according to the feedback.

[denis256]

Proposed functions don't follow existing patterns:

• no *E function which check for errors
• t testing.TestingT not passed

Go mod don't have all changes commited

$ git status
On branch feat/add-remote-repo-support-in-helm
nothing to commit, working tree clean
$ go mod tidy
$ git status
On branch feat/add-remote-repo-support-in-helm
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   go.mod

[denis256]

in

[denis256]

Approach looks fine, some wording updates and comments on functions should be updated

[jguionnet]

@denis256. Thank you for your thorough review and for catching all my mistakes. I fixed it according to your latest feedback.