Fix race condition on proxy server

[Nathan-SL]

Usage related changes

• Improves the behavior of the proxy server when making multiple calls to fetch pre-deployed accounts.
• Closes Proxy server failure on multiple calls: starknet-venv-proxy exited with code=1 while running  #373

Development related changes

• Adds a function to ensure proper assignment of a free port.

Checklist:

• Formatted the code
• No linter errors + tried to avoid introducing linter warnings
• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Updated the test directory (with a test case consisting of network.json, hardhat.config.ts, check.ts)
• Linked issues which this PR resolves
• Created a PR to the plugin branch of starknet-hardhat-example:Add utility func to get n pre-deployed accounts starknet-hardhat-example#112
  • Modified test.sh to use my example repo branch
  • Restored test.sh to to use the original branch (after the example repo PR has been merged)
• All tests are passing (for external contributors who don't have access to the CI/CD pipeline)