Check the detection of infinite recursion.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:extend file="extended1.tpl">
<opt:snippet name="foo">
FOO-VALUE1
</opt:snippet>
</opt:extend>

>>>>templates/extended1.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:extend file="test.tpl">
<opt:snippet name="foo">
FOO-VALUE2
</opt:snippet>
</opt:extend>

>>>>expected.txt
Opt_InheritanceRecursion_Exception