Instead of letting each nmcli run write an individual .expected file,
combine the output of multiple runs in one file (per test).

Advantages:

- since there is a very large number of tests, having a file for each
  tests is cumbersome. For example, since they are all added to
  $(EXTRA_DIST) (and since we use non-recursive make), autoconf can
  easily hit a length limit when processing all the file names.

- previously, whenever we added tests, all .expected files shifted
  and the diff was large. Now, there is a chance that the diff is
  smaller and more accurate.