tests/tcg: update licenses to GPLv2 as intended

tests/tcg: update licenses to GPLv2 as intended

My default header template is GPLv3 but for QEMU code we really should
stick to GPLv2-or-later (allowing others to up-license it if they
wish). While this is test code we should still be consistent on the
source distribution.

I wrote all of this code so its not a problem. However there remains
one GPLv3 file left which is the crt0-tc2x.S for TriCore.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-2-alex.bennee@linaro.org>