Optional dlopen support for drivers

tests/testutils.c

@@ -151,6 +151,9 @@ void virtTestCaptureProgramExecChild(const char *const argv[],
     int stderrfd = -1;
     const char *const env[] = {
         "LANG=C",
+#if WITH_DRIVER_MODULES
+        "LIBVIRT_DRIVER_DIR=" TEST_DRIVER_DIR,
+#endif
         NULL
     };