testhelper: move methods to internal/librarian/rust

[julieqiu]

Once #3376 is completed, most of the testhelper methods can be moved to _test.go files inside internal/librarian/rust.

[ldetmer]

@julieqiu all of the functions in testhelper are still being used outside of a single test. Is this a larger ticket to try to refactor tests so that they don't rely on these testhelper functions or can we close this as won't fix?

Here is a summary of my findings:

• RequireCommand: Used in 14 different test files across internal/git, internal/librarian, and
  internal/sidekick.
• SetupForVersionBump: Used in internal/librarian and internal/git
• ContinueInNewGitRepository: Used by SetupForVersionBump
• AddCrate: Used indirectly for SetupForVersionBump
• SetupRepo: Used in internal/librarian/release_test.go and internal/git/git_test.go.
• SetupRepoWithChange: Used in internal/librarian/publish_test.go, internal/librarian/release_test.go,
  and internal/git/git_test.go.
• CloneRepository: Used in internal/librarian/publish_test.go and internal/librarian/release_test.go.

[julieqiu]

Let's close this for now and we can revisit later.