Default_tasks: gcov:all doesn't work

[Bau21180]

If you add "gcov:all" as default task, it compiles the relevant files with coverage, but no coverage report is created.

  # tweak the way ceedling handles automatic tasks
  :build_root: build
  :test_file_prefix: test_
  :default_tasks:
    - gcov:all

A colleague of mine hadn't the gcovr package installed. He typed "ceedling" and it executed well without generating a coverage report. He typed "ceedling gcov:all" and got an error message that "gcovr" isn't recognized. Hope this helps to identify the location of this issue.