test: make fee tests robust

[itegulov]

What ❔

A bunch of small improvements to make fee tests robust:

• Upgrade ethers
• Catch retryable networking issues in ethers.FetchRequest
• Destroy providers on context teardown to avoid econnreset
• Retry fee-related issues that can arise when transaction is rejected by mempool

Why ❔

Fee tests are disruptively flaky right now

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.

[itegulov]

Verified that this PR passes fee tests 10 times in a row (job). Revert tests failed but its for an unrelated reason (its flaky when batch execution is behind, as happens when fee tests are ran 10 times in a quick succession).