buildtest-TESTS is a phony target and does nothing which results in a
do_install error since the tests aren't built. Since there isn't
a suitable make target but the number of tests are small, hardcode
the two to build to unbreak the build when ptest is enabled.
(From OE-Core rev: 5dd8653fdcda5e0e8b4f3c37a46f357bc97ec66c)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>