lib/igt_aux: Improve wait_for_keypress helper a bit

- Use keys in just one env variable to enable/disable it.
- Add an informational message so that the users knows when to press
  the key (more useful over ssh than when run on the terminal ofc).
- Improve the documentation so that it's clearer how to use this
  when running tests.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
3 files changed