Skip gdb.mi/list-thread-groups-available.exp if no xml support

Skip gdb.mi/list-thread-groups-available.exp if no xml support

I see the following test fail in gdb (configured --with-expat=no),

-list-thread-groups --available^M
&"warning: Can not parse XML OS data; XML support was disabled at compile time\n"^M
^error,msg="Can not fetch data now."^M
(gdb) ^M
FAIL: gdb.mi/list-thread-groups-available.exp: list available thread groups (unexpected output)

This patch skips it if XML parsing in GDB is disabled, like what you did
in gdb.mi/mi-info-os.exp.

gdb/testsuite:

2017-11-03  Yao Qi  <yao.qi@linaro.org>

	* gdb.mi/list-thread-groups-available.exp: Skip it if XML parsing
	in GDB is disabled.