selftests/ftrace: install test.d

The ftrace test requires the directory test.d and all of it's contents to be
present during execution. Use TEST_DIRS to ensure this is copied to the
INSTALL_PATH.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>