package/tinyxml2: fix cmake warning

CMake complains about Policy CMP0037 that reserve some
target name such "test".
So remove the warning by renaming test to xmltest.

Backport an upstream patch:
0b2c22bd46be95b08f234b8b4cb262f72ab3f0e8

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>