Misc tasks towards large-scale linear and nonlinear problems with GridapDistributed + GridapPETSc

## High-priority
- [x] Refactor `PetscNonlinearSolver`. Issue: #45 @amartinhuertas https://github.com/gridap/GridapPETSc.jl/pull/53
- [x] Fix `copy!`. Issue #50 @amartinhuertas https://github.com/gridap/GridapPETSc.jl/pull/53
- [ ] Refactor mpi tests with a TestApp and try to put as much drivers as possible in the same mpi run to reuse JIT compilation cache. Essentially, all test requiring the same number of procs can be in the same run. This should reduce the CI time significantly
- [x] Garbage collection of petsc objects. Issue #41. To fully fix this, it is likely that we need to fix #51 first.
- [ ] https://github.com/gridap/GridapPETSc.jl/issues/45#issue-1040039821

It would be nice to have all functionality working in native sequential (just as before), as well as in distributed sequential and distributed mpi mode.