Problem: eth batch tx is not tested

[yihuang]

Proposal: add tests for eth batch tx feature

Current behavior: it's verified manually and in cronos integration test, but not in ethermint repo.

Desired behavior: good to add integration test in ethermint repo too.

Use case: [Why is this important (helps with prioritizing requests)]

Requests may be closed if we're not actively planning to work on them.

[fedekunze]

we should also add unit tests to the RPC util functions that were introduced in your PR

[github-actions]

This issue is stale because it has been open 45 days with no activity. Remove Status: Stale label or comment or this will be closed in 7 days.