Increase timeout in gdb.mi/list-thread-groups-available.exp

Increase timeout in gdb.mi/list-thread-groups-available.exp

Running

    make check-read1 TESTS="gdb.mi/list-thread-groups-available.exp"

on my machine results in timeout failures.  Running it while having
`tail -F testsuite/gdb.log` on the side shows that the test is never
really blocked, it is just slow at consuming the large output generated
by `-list-thread-groups --available` (which lists all the processes on
the system).

If I increase the timeout to a large value, the test passes in ~30
seconds (compared to under 1 second normally).

Increase the timeout for the particular mi_gdb_test that is long to
execute under read1.  The new timeout value is a bit arbitrary.  The
default timeout is 10 seconds, so I set the new timeout to be
"old-timeout * 10", so 100 seconds in the typical case.

gdb/testsuite/ChangeLog:

	PR gdb/24863
	* gdb.mi/list-thread-groups-available.exp: Increase timeout for
	-list-thread-groups --available test when running under
	check-read1.