Make test try to toggle the speculation state.

It looks like a very recent kernel update has made `PR_SET_SPECULATION_CTRL` succeed
when you try to change an immutable state to the state it currently has.

src/test/prctl_speculation_ctrl.c

@@ -31,6 +31,10 @@ int main(void) {
                                 PR_SPEC_ENABLE, 0, 0) &&
                     errno == EPERM);
       }
+    } else if (ret & PR_SPEC_ENABLE) {
+      test_assert(-1 == prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS,
+                              PR_SPEC_DISABLE, 0, 0) &&
+                  errno == ENXIO);
     } else {
       test_assert(-1 == prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS,
                               PR_SPEC_ENABLE, 0, 0) &&