gdb.mi/list-thread-groups-available.exp: read entries one by one instead of increasin...

gdb.mi/list-thread-groups-available.exp: read entries one by one instead of increasing timeout

Commit 580f1034 ("Increase timeout in
gdb.mi/list-thread-groups-available.exp") changed
gdb.mi/list-thread-groups-available.exp to significantly increase the
timeout, which was necessary for when running with make check-read1.

Pedro suggested a better alternative, which is to use gdb_test_multiple
and consume one entry at a time.  This patch does that.

gdb/testsuite/ChangeLog:

	* gdb.mi/list-thread-groups-available.exp: Read entries one by
	one instead of increasing timeout.

Change-Id: I51b689458503240f24e401f054e6583d9172ebdf