Test MASP inverse conversions (backport #4193)

[mergify]

Describe your changes

Adds tests for the MASP inverse conversions.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

---

This is an automatic backport of pull request #4193 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 294e44c has failed:

On branch mergify/bp/maint-libs-0.46/pr-4193
Your branch is up to date with 'origin/maint-libs-0.46'.

You are currently cherry-picking commit 294e44c4d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   crates/shielded_token/src/masp/shielded_wallet.rs

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally