core: svolume tests should generate realistic random volume data

assuming RAND_MAX is around 1<<31, rand() >> 1 generates large numbers as
random volume data; these likely causes saturated sample values after
applying the volume function -- not a good test

src/pulsecore/svolume_orc.c

@@ -75,7 +75,7 @@ static void run_test(void) {
     memcpy(samples_orig, samples, sizeof(samples));
 
     for (i = 0; i < CHANNELS; i++)
-        volumes[i] = PA_CLAMP_VOLUME(rand() >> 1);
+        volumes[i] = PA_CLAMP_VOLUME(rand() >> 15);
     for (padding = 0; padding < PADDING; padding++, i++)
         volumes[i] = volumes[padding];