Update outdated testing instructions

[KipCrossing]

In this PR, the section Running the test suite is updated:

• ANVIL_FORK_URL is changed to VITE_ANVIL_FORK_URL to match above
• clarity the the RPC service provider should be mainnet
• Remove pnpm anvil step. It doesn't seem to be required
• Add instructions to re-run with f in case it fails the first time.

Still waiting for #996 to be concluded tho.

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PR-Codex overview

Detailed summary

• Updated the variable name ANVIL_FORK_URL to VITE_ANVIL_FORK_URL in .github/CONTRIBUTING.md.
• Added a note about running tests in watch mode using pnpm test.
• Added a note about rerunning failed tests by pressing f.
• Added a note about the importance of adding test cases for new features or bug fixes.

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