CommonSeeder test needs a white-box testing mechanism to replace the default entropy source

test/jdk/sun/security/provider/SecureRandom/CommonSeeder.java

This test currently uses reflection to hack into a Field object of a static final field to "non-final". It needs a white-box testing mechanism to replace this reflection hack.

    static void setDefaultSeeder(EntropySource es) throws Exception {
        Field f = AbstractDrbg.class.getDeclaredField("defaultES");
        f.setAccessible(true); // no more private
        Field f2 = Field.class.getDeclaredField("modifiers");
        f2.setAccessible(true);
        f2.setInt(f, f2.getInt(f) - Modifier.FINAL); // no more final
        f.set(null, es);
    }