Add several "quit with live inferior" tests

In my multi-target branch, I had managed to break GDB exiting
successfuly in response to "quit" or SIGHUP/SIGTERM when:

 - you're debugging with "target extended-remote",
 - have more than one inferior loaded in gdb, some running, and at
   least one not running, and,
 - quit gdb with the inferior that is not running yet selected.

The testsuite still passed cleanly anyway.  I only noticed because I
was left with a bunch of core dumps in the gdb/testsuite/ directory --
the testsuite infrastructure closes GDB's pty after running each
testcase, which results in GDB getting a SIGHUP and should make GDB
exit gracefully.  If GDB crashes at that point though, there's no
indication about it in gdb.sum/gdb.log.

This commit adds a multitude of tests exercising quitting GDB with
live inferiors, some of which would have caught the problem.

gdb/testsuite/ChangeLog:
2017-10-17  Pedro Alves  <palves@redhat.com>

	* gdb.base/quit-live.c: New file.
	* gdb.base/quit-live.exp: New file.

1 files changed, 27 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/quit-live.c b/gdb/testsuite/gdb.base/quit-live.c
new file mode 100644
index 0000000000..d29fd25bd0
--- /dev/null
+++ b/gdb/testsuite/gdb.base/quit-live.c
@@ -0,0 +1,27 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+
+int
+main ()
+{
+  int secs = 30;
+
+  while (secs--)
+    sleep (1);
+}