Use initlog and mpi_initlog in tests

[masterleinad]

Although there are already initlog and mpi_initlog to unify the initialization of tests, a manual setup (including std::ofstream logfile("output");) is used in a lot of tests.

[bangerth]

Yes, that's one of those incomplete conversions that are just incredibly difficult to complete because we have sooo many tests. If someone wanted to make progress, that would be fantastic.

[bangerth]

Or if someone wanted to script this somehow.