random_positive_definite_matrix.m: add condmax parameter

random_positive_definite_matrix.m: add condmax parameter

We are getting some rare test failures due to these random
positive-definite matrices having huge condition numbers.
In the PCGM case...

  * cond(A)=52879791.988025, diff=0.014173772186
  * cond(A)=4545512.611295, diff=0.000072462950
  * cond(A)=5394929.793492, diff=0.000019629413
  * etc.

To avoid that problem consistently, we add a "condmax" parameter to
the random matrix generator. The default (1000) is conservative to
ensure that the tests pass even when we are not preconditioning.