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

Prior to commit eac646ea49 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: eac646ea49
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

tests/qemucapsprobe.c

@@ -48,7 +48,7 @@ main(int argc, char **argv)
     virQEMUCaps *caps;
     virArch host;
     virArch guest;
-    const char *mock = VIR_TEST_MOCK("qemucapsprobe");
+    g_autofree char *mock = g_strdup_printf("%s/%s", abs_builddir, VIR_TEST_MOCK("qemucapsprobe"));
 
     if (!virFileIsExecutable(mock)) {
         perror(mock);