tests/prng.c is GPL-2.0-only

I discovered that tests/prng.c file has a different license from other tests and code:

 * This program is distributed under the GNU General Public License, version 2.
 * A copy of this license is included with this source.

Is it a mistake, or a known feature? If it is expected, would it be possible to add a copy of GPL-2.0 license into the source archive? COPYING file quotes LGPL-2.0.