Fix convergence result for trivial solution

[Samev]

Due to changes in #261 a system which is already solved will incorrectly be reported as AMGX_ST_NOT_CONVERGED. This commit adjusts this behavior and also adds a test case for the scenario.

[Samev]

Can't assign you as a reviewer @mattmartineau so a ping will have to suffice.

[mattmartineau]

Nice - thanks for the contribution!

I'll test.