tests/preconditioned_conjugate_gradient_method_tests.m: make reliable?

tests/preconditioned_conjugate_gradient_method_tests.m: make reliable?

This was ultimately made reliable by the new "condmax" parameter to
random_positive_definite_matrix(), but there are some additional
tweaks afterwards:

  * Reorder / rename some stuff to improve readability
  * Use a higher condmax than default, since we are preconditioning
  * Pass the full "M" matrix to pcg(), to more closely match what
    our own PCG function is doing.

The tests "always" pass now.