test(testnode): configure custom min gas price and fix chain id

[mergify]

Context: celestiaorg/celestia-node#3453 (comment)

Fix testnode so that it is possible to query the local min gas price again. I plan on backporting this to v2.x to fix celestia-node integration tests which query local min gas price.

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This is an automatic backport of pull request #3680 done by Mergify.

[mergify]

Cherry-pick of 5b27eba has failed:

On branch mergify/bp/v2.x/pr-3680
Your branch is up to date with 'origin/v2.x'.

You are currently cherry-picking commit 5b27eba3.
  (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:   pkg/user/tx_client_test.go
	modified:   test/util/testnode/config.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   app/test/testnode_test.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