Avoid load(testcase) for installing test helper apps

Avoid load(testcase) for installing test helper apps

Make test projects declare TEST_HELPER_INSTALLS rather than calling a
function exported by testcase.prf.  load(testcase) may be unsafe, as
testcase.prf should be processed after default_post.prf.

Fixes silent disabling of various autotests.

Change-Id: I56b35ffd653a637ad5ab18d64dd1a1edadfac59f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Kurt Korbatits  <kurt.korbatits@nokia.com>