Improve robustness of tests (part1)

[jmachado-amd]

This PR adds the first part of a series of changes that are meant to improve the robustness of some eigensolvers' tests. It tackles the case in which a subset of the eigenvalues is requested.

The reference eigenvalues are still being computed with LAPACK; tests' inputs will be updated in a future PR.

[Documentation will follow shortly.]