tests: qemucapsprobe: Fix construction of path to libqemucapsprobemock.so

tests: qemucapsprobe: Fix construction of path to libqemucapsprobemock.so

Prior to commit eac646ea490e66 VIR_TEST_MOCK included the path to the
build directory, but the code was not fixed after VIR_TEST_MOCK was
changed resulting in the following failure when attempting to probe
capaibilities:

 $ ./tests/qemucapsprobe /path/to/qemu/qemu-system-x86_64 > out
 libqemucapsprobemock.so: No such file or directory

Fix the construction of the path to the mock library by concatenating it
back with the absolute path to the build directory.

Fixes: eac646ea490e66500609585047f0d800e3645d6b
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>