python3: Fix failing sysconfig.py test on x86(64 bit) using lib64 as baselib by updat...

python3: Fix failing sysconfig.py test on x86(64 bit) using lib64 as baselib by updating test_sysconfig for posix_user purelib

Steps to trigger the failed test:
Edit local.conf to add something as follows:
    BASELIB = "lib64"
    IMAGE_INSTALL:append = " python3-tests".
bitbake core-image-sato
runqemu qemux86-64 nographic slirp
Reproducer:
    $python3 -m test test_sysconfig

sysconfig.py use platlibdir for purelib. Update
test_sysconfig.test_user_similar() for the posix_user
scheme: "purelib" doesn't use sys.platlibdir.

Signed-off-by: Wentao Zhang <wentao.zhang@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>