selftests: copy TEST_DIRS to INSTALL_PATH

Loop over all TEST_DIRS and recursively copy them to the INSTALL_PATH. Tests
such as ftrace require a directory and all of it's contents to execute the
test properly, thus these directories and files need to be copied when we
perform an install.

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>