tests: Avoid double linking some libraries

tests: Avoid double linking some libraries

The problem is, since 614581f32b domaincapstest is linked with
$(LDADDS) by default. Then, since 94e3f23e8a7 the test may be
conditionally linked with $(qemu_LDADDS) which already contains
$(LDADDS). And some linkers doesn't cope with this nicely:

  CCLD     domaincapstest
../src/libvirt_probes.o:(.probes+0x0): multiple definition of `libvirt_event_poll_add_handle_semaphore'
../src/libvirt_probes.o:(.probes+0x0): first defined here
../src/libvirt_probes.o:(.probes+0x2): multiple definition of `libvirt_event_poll_update_handle_semaphore'
../src/libvirt_probes.o:(.probes+0x2): first defined here
../src/libvirt_probes.o:(.probes+0x4): multiple definition of `libvirt_event_poll_remove_handle_semaphore'
../src/libvirt_probes.o:(.probes+0x4): first defined here
../src/libvirt_probes.o:(.probes+0x6): multiple definition of `libvirt_event_poll_dispatch_handle_semaphore'
../src/libvirt_probes.o:(.probes+0x6): first defined here

And so on.

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