[gdb/testsuite] Fix tcl error in gdb.mi/list-thread-groups-available.exp

When running gdb.mi/list-thread-groups-available.exp, we get:
...
Running gdb.mi/list-thread-groups-available.exp ...
ERROR: tcl error sourcing gdb.mi/list-thread-groups-available.exp.
ERROR: Too many arguments to gdb_test_multiple
...

The problem is that the gdb_test_multiple call has as last argument a
$mi_gdb_prompt, which is no longer supported syntax since 590003dc0e
"[gdb/testsuite] Add -lbl option in gdb_test_multiple".

Fix this by using the new -prompt syntax.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-03  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/list-thread-groups-available.exp: Use -prompt syntax for
	gdb_test_multiple call.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-03  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.mi/list-thread-groups-available.exp: Use -prompt syntax for
+	gdb_test_multiple call.
+
 2020-03-03  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.mi/mi-sym-info.exp: Add missing -prompt "$mi_gdb_prompt$" to

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

@@ -56,7 +56,9 @@ set process_entry_re "{${id_re},${type_re}(,$description_re)?(,$user_re)?(,$core
 
 # The list can be long, so read entries one by one to avoid hitting the
 # timeout (especially when running with check-read1).
-gdb_test_multiple "-list-thread-groups --available" "list available thread groups" {
+set cmd "-list-thread-groups --available"
+set test "list available thread groups"
+gdb_test_multiple $cmd $test -prompt "$mi_gdb_prompt" {
     -re "\\^done,groups=\\\[" {
 	# The beginning of the response.
 	exp_continue
@@ -71,7 +73,7 @@ gdb_test_multiple "-list-thread-groups --available" "list available thread group
 	# The last entry.
 	pass $gdb_test_name
     }
-} $mi_gdb_prompt
+}
 
 # List specific processes, make sure there are two entries.
 set spawn_id_1 [remote_spawn target $binfile]