ptest.bbclass: add cleandirs flag to do_install_ptest_base
The do_install_ptest_base task should have cleandirs flag, just like
the do_install task. The ${D}${PTEST_PATH} directory should be cleaned,
Otherwise, there would be similar errors like below if the
do_install_ptest_base task is rerun.
ln: failed to create symbolic link `xxx': File exists
