[gdb/testsuite] Make gdb_test message more informative in multi-term-settings.exp

[gdb/testsuite] Make gdb_test message more informative in multi-term-settings.exp

This racy fail message, reported in PR24929:
...
FAIL: gdb.multi/multi-term-settings.exp: inf1_how=attach: inf2_how=attach: \
  stop with control-c
...
does not make clear which gdb_test fails here:
...
    if {$expect_ttou} {
       gdb_test "" "Quit" "stop with control-c"
    } else {
       gdb_test "" "received signal SIGINT.*" "stop with control-c"
    }
...

Fix this by making the gdb_test message argument more informative.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-08-22  Tom de Vries  <tdevries@suse.de>

	* gdb.multi/multi-term-settings.exp (coretest): Make gdb_test messages
	more informative.