Fix build with XML::LibXML 2.0202 Perl module

Version 2.0202 of XML::LibXML changes the default behaviour regarding
external entities. This results empty tests_list.h and tests_list.c
generated by generate_test_list.pl. By enabling explicitly to expand
entities the script works correctly. The modified initalization of
LibXML is compatible with older versions of the module.

Change-Id: Icb951542e70870bddda44d03030eb1fe49689374
Signed-off-by: Imre Kis <imre.kis@arm.com>