input: Write a python function to generate REXI benchmarks. Use the provided code and include the following assertions to test the function:
  assert len(get_rexi_benchmarks(mock_jg)) == 4
  assert isinstance(get_rexi_benchmarks(mock_jg)[0], dict)
  assert "rexi_method" in get_rexi_benchmarks(mock_jg)[0]
  assert "rexi_files_coefficients" in get_rexi_benchmarks(mock_jg)[0]
output: def get_rexi_benchmarks(mock_jg):
      benchmarks = []
  
      rexi_methods = ['phi0', 'phi1', 'phi2', 'phi3']
      rexi_files_coefficients = ['coefficients_phi0.txt', 'coefficients_phi1.txt', 'coefficients_phi2.txt', 'coefficients_phi3.txt']
  
      for method, coeff_file in zip(rexi_methods, rexi_files_coefficients):
          benchmark = {
              'rexi_method': method,
              'rexi_files_coefficients': coeff_file
          }
          benchmarks.append(benchmark)
  
      return benchmarks