Use LGPLv2.1 license for hbtest3

The license was supposed to be the same as the other code has,
i.e. "LGPLv2.1". Due to copy paste error "LGPLv2" got applied.

[tests] Use LGPLv2.1 license for hbtest3
Set COPYING to GNU Lesser General Public License, version 2.1

Already in the initial commit every C and python source file refers to
LGPL v2.1. There is no code that refers to GPL.

Replaced existing COPYING file that contained GPL license with LGPL v2.1
license text retrieved from:
  https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt

[libiphb] Set COPYING to GNU Lesser General Public License, version 2.1
Merge pull request #8 from spiiroin/licenses

Use LGPLv2.1