tests: Put a mock library at the start of LD_PRELOAD

This fixes vircgrouptest when run in a sandbox which already overrides
open() and others.

tests/testutils.h

@@ -87,8 +87,9 @@ int virtTestMain(int argc,
                 perror(lib);                                            \
                 return EXIT_FAILURE;                                    \
             }                                                           \
-            if (virAsprintf(&newenv, "%s%s%s", preload ? preload : "",  \
-                            preload ? ":" : "", lib) < 0) {             \
+            if (!preload) {                                             \
+                newenv = (char *) lib;                                  \
+            } else if (virAsprintf(&newenv, "%s:%s", lib, preload) < 0) {   \
                 perror("virAsprintf");                                  \
                 return EXIT_FAILURE;                                    \
             }                                                           \