add test for FlatIO port ordering (backport #4113)

[mergify]

This is just a unit test. I thought going into writing the test that FlatIO maybe was not working as we intended, but i think it is just general misunderstanding of how Record and Bundle treat their elements.

This might be more of a test of CIRCT's lowering ABI, but firtool internally considers the port order part of the contract (not just the name of the signals), so check that we are doing what is sane here.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Added a unit test for FlatIO Ordering being maintained

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

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This is an automatic backport of pull request #4113 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c3c9979 has failed:

On branch mergify/bp/5.x/pr-4113
Your branch is up to date with 'origin/5.x'.

You are currently cherry-picking commit c3c997939.
  (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:   src/test/scala/chiselTests/experimental/FlatIOSpec.scala

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

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