ptimer-test: do not link to libqemustub.a/libqemuutil.a

This test provides its own mocks, so do not use the "standard"
stubs in libqemustub.a or the event loop implementation in
libqemuutil.a.

This is required on OS X, which otherwise brings in qemu-timer.o,
async.o and main-loop.o from libqemuutil.a.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
