chore(client): add more tests

[weisdd]

• client:
  • extracted logic around fetching admin credentials from environment variables from getAdminCredentials into getContainerEnvCredentials to simplify testing and improve readability;
  • added tests cases covering getContainerEnvCredentials;
  • added a t.Cleanup call to createFileWithContent, so we don't have to explicitly remove the files inside tests anymore.