ptest-runner: pass libdir to DEFAULT_DIRECTORY when compiling

ptest-runner: pass libdir to DEFAULT_DIRECTORY when compiling

Ptest data are installed to PTEST_PATH ?= "${libdir}/${BPN}/ptest".
When libdir isn't "/usr/lib", it will cause "No ptests found" error
when running "ptest-runner" command. Here pass libdir to
DEFAULT_DIRECTORY when compiling to avoid that error.

Need update ptest-runner source code to get the new commit:
<ptest-runner: make DEFAULT_DIRECTORY be able to be defined when compiling>

(From OE-Core rev: 1447afd62035f42344a32a487bec408e6563d5f6)

Signed-off-by: Li Zhou <li.zhou@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>