classes/testimage: add support for finding tests in other layers

classes/testimage: add support for finding tests in other layers

A layer can add tests in lib/oeqa/runtime (provided it extends BBPATH as
normal) and enable them with TEST_SUITES_append = " testname". Test
module names shouldn't collide though.

(From OE-Core rev: e1e347a2d509303e1c566450b0f2b485d3d6629f)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>