test: update block proposer in testing (backport #7430)

[mergify]

Description

Currently, test chains carry the same block proposer for every block. This can lead to a situation where the block proposer isn't part of the validator set, such as after being jailed. This can cause the validation of validator sets to fail in CometBFT v0.38.12, see ValidateBasic.

This PR addresses the issue by incrementing the validator proposer priority in test chains, ensuring that the block proposer is updated in line with changes to the validator set.

closes: #XXXX

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against the correct branch (see CONTRIBUTING.md).
• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Code follows the module structure standards and Go style guide.
• Wrote unit and integration tests.
• Updated relevant documentation (docs/).
• Added relevant godoc comments.
• Provide a conventional commit message to follow the repository standards.
• Include a descriptive changelog entry when appropriate. This may be left to the discretion of the PR reviewers. (e.g. chores should be omitted from changelog)
• Re-reviewed Files changed in the GitHub PR explorer.
• Review SonarCloud Report in the comment section below once CI passes.

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This is an automatic backport of pull request #7430 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 64f33e0 has failed:

On branch mergify/bp/release/v8.5.x/pr-7430
Your branch is up to date with 'origin/release/v8.5.x'.

You are currently cherry-picking commit 64f33e09.
  (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)

Changes to be committed:
	modified:   CHANGELOG.md
	modified:   testing/chain_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   testing/chain.go

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

[colin-axner]

Thank you @DimitrisJim for picking up mergify's slack!