Install the test programs by default so that they can be packaged.

Tested with the testdisplay test so that it still runs after the
modifications as it depends on a data file to be present. Need to
pass -r option to enable QR code display on success (PNG data file).

Packaging is useful when building a complete software stack for a
DUT from scratch. This should bring us closer to achieving a
built-from-scratch testing workflow.

Package maintainers can always decide to ignore the installed files.

v2:
- Install more tests including scripts and their data

v3:
- Add clarification to commit message about why we do this.
  (Chris Wilson & Thomas Wood)
- Change libexec into pkglibexec to comply to standard
  (Thomas Wood)
- Do not install $(common_files). (Thomas Wood)
- Make it really obvious the installed files are tests by using
  tests directory name to avoid any confusion with packagers.

v4:
- Fixed commit message.

v5:
- Add file locator helper to retain backwards compatibility.
  (Thomas Wood)
- Test with testdisplay -r option that draws the .png file.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>