Test bits/step-51: Improve output

This allows numdiff to catch small roundoff errors.
Test umfpack/umfpack_04: Avoid printing exact number of iterations

This commit introduces a new macro for tests to test an iterative solve to
convergence within a given number of iteration steps. Due to roundoff
errors the exact number doesn't have to be stable...
port other tests to new macro
Merge pull request #361 from tamiko/improve_test_stability

Improve test stability