Annotate unused parameters in tests

Compiling the tests with -Wextra results to lots of warnings for unused
parameters. Annotate these cases with the `unused' attribute to avoid
the warnings.
diff --git a/tests/test-async-sig.c b/tests/test-async-sig.c
index 5e90690..296d58d 100644
--- a/tests/test-async-sig.c
+++ b/tests/test-async-sig.c
@@ -158,7 +158,7 @@
 }
 
 int
-main (int argc, char **argv)
+main (int argc, char **argv __attribute__((unused)))
 {
   struct sigaction act;
   long i = 0;