runtime/cases/ptest.py: do not require ptest-pkgs in IMAGE_FEATURES; run only when...

runtime/cases/ptest.py: do not require ptest-pkgs in IMAGE_FEATURES; run only when ptest-runner is availalble;

Previously the test would execute only when all available ptests
for packages in the image were installed; some of those tests may
be broken, never finish, take a very long time or simply irrelevant
to the user who wants to check ptests of only a few specific packages,
and does so by listing them explicitly via IMAGE_INSTALL_append or similar.

Presence of ptest-runner means there is at least one ptest package installed
as they pull it in via a class dependency; ptest-runner is not generally
installed otherwise.

(From OE-Core rev: e07a2b9c2b08a465baeaaca86461e07817f84a52)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>