virportallocatortest: Run on linux only

virportallocatortest: Run on linux only

After the rework of mocking of our tests there's the
virportallocator test failing to link on mingw. Well, it's the
mocking library actually:

../gnulib/lib/.libs/libgnu.a(bind.o): In function `rpl_bind':
/home/jenkins/libvirt-mingw/build32/gnulib/lib/../../../gnulib/lib/bind.c:33: multiple definition of `rpl_bind'
.libs/virportallocatormock_la-virportallocatormock.o:/home/jenkins/libvirt-mingw/build32/tests/../../tests/virportallocatormock.c:79: first defined here

I've no idea why this matters to mingw and does not to others.
Nevertheless, if we make the test linux only the problem goes
away.

Apparently, our test for RTLD_NEXT is not sufficient because
mingw32 defines it. Lets put aside for a while fact that it has
the same value as RTLD_DEFAULT which by description has different
meaning, shall we?

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>