tests: License all files to GPLv2+

A few files were still licensed under the LGPLv2+.

These files were authored and modified by myself, Robin Getz, Lars-Peter
Clausen and Matej Kenda; all parties gave their consent for the
relicense in the following github thread:
#924

Signed-off-by: Paul Cercueil <[email protected]>

Author: pcercuei

tests/gen_code.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: LGPL-2.1-or-later
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * libiio - Library for interfacing industrial I/O (IIO) devices
  *

tests/gen_code.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 /*
  * libiio - Library for interfacing industrial I/O (IIO) devices
  *

tests/iio_stresstest.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: LGPL-2.1-or-later
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * libiio - Library for interfacing industrial I/O (IIO) devices
  *