Fix a test.

We have recently allowed that one can set a vector to zero, even if it has
ghost elements. But this test verified that we throw an exception in this
case. Remove this one test.
Also ensure that we have ML as part of Trilinos.
Merge pull request #208 from bangerth/augment-trilinos-detection

Also ensure that we have ML as part of Trilinos.
Merge pull request #207 from bangerth/fix-test

We have recently allowed that one can set a vector to zero, even if it has
ghost elements. But this test verified that we throw an exception in this
case. Remove this one test.