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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+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.
+
 2017-11-03  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.python/py-thrhandle.exp: Skip it if python is not

gdb/testsuite/gdb.mi/list-thread-groups-available.exp

@@ -20,6 +20,12 @@ set MIFLAGS "-i=mi"
 
 standard_testfile
 
+# Support for XML is needed to run this test.
+if [gdb_skip_xml_test] then {
+    unsupported "list-thread-groups-available.exp"
+    return -1
+}
+
 if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
     untested "failed to compile"
     return -1