Improve gmx::test::CommandLine.

Improve gmx::test::CommandLine.

- Make the class properly copyable for easier use in tests that need to
  manipulate it.
- Make the class manage its memory using std::vector and prevent memory
  leaks if the argv array is modified outside the class (the internal
  state of the class is still not correctly maintained, though).
- Add methods to append arguments.

Needed for #920, but split from the main change to keep it smaller.

Change-Id: I66ebe9d48936d35bce9e0b4fc1d6246d3d5ec07d