ptest.bbclass: use BPN in PTEST_PATH for multilib

ptest.bbclass: use BPN in PTEST_PATH for multilib

Use BPN instead of PN in PTEST_PATH for multilib builds,
or we get two directories for a package in libdir which
doesn't make sense, e.g.

$ ls /usr/lib/*coreutils
/usr/lib/coreutils:
libstdbuf.so

/usr/lib/lib32-coreutils:
ptest

(From OE-Core rev: 0e949b7bfc4207aba0e3c4d12b76cc1f1815470d)

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>