[gdb/testsuite] Require allow_xml_test in gdb.base/gcore.exp

I build gdb without xml support, and with test-case gdb.base/gcore.exp ran
into:
...
(gdb) core $outputs/gdb.base/gcore/gcore.test
warning: Can not parse XML target description; \
  XML support was disabled at compile time
  ...
(gdb) FAIL: $exp: corefile restored general registers
...

Fix this by requiring allow_xml_test.