Extend and/or improve test for eigensolvers (evp, gevp, tridsolver)

[albestro]

Despite an important part was missing in the implementation of the tridiagonal solver (see #960), tests were not able to spot the problem.

We are aware that current unit tests are not covering well the domain of possible usages, but we should look forward to improve them.

Reporting here an idea that has been shared during meetings, at least as a starting point for reasoning: we might look at ScaLAPACK tests for corner cases and see if we can adapt them somehow to our library.