Fix 'gdb.base/quit.exp hangs forever' if the test fails

The [wait -i $gdb_spawn_id] in the test is dangerous in the sense that
it won't be subject to timeout logic.  So if GDB fails quiting, this
testcase hangs forever, hanging the test run with it.  See:
  https://sourceware.org/ml/gdb-patches/2016-10/msg00728.html

Instead of 'wait'ing directly, use gdb_test_multiple and expect 'eof'.

Tested that the testcase no longer hangs by hacking the test to send
"info threads" instead of "quit".

Tested with
  --target_board={unix, native-gdbserver,native-extended-gdbserver}
and tested with
  --host_board=local-remote-host
as well.

gdb/testsuite/ChangeLog:
2017-10-20  Pedro Alves  <palves@redhat.com>

	* gdb.base/quit.exp: Use gdb_test_multiple and expect 'eof' before
	'wait -i'.  Use gdb_assert and remote_close.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-10-20  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/quit.exp: Use gdb_test_multiple and expect 'eof' before
+	'wait -i'.  Use gdb_assert and remote_close.
+
 2017-10-19  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.linespec/ls-errs.exp (do_test): Update comment, use line

gdb/testsuite/gdb.base/quit.exp

@@ -23,13 +23,15 @@ gdb_test "quit()" "A syntax error in expression, near .*" \
     "quit with syntax error"
 
 # Test that an expression can be used to set the error code.
-send_gdb "quit 22+1\n"
+
+set test "quit with expression"
+gdb_test_multiple "quit 22+1" $test {
+    eof {
 	set result [wait -i $gdb_spawn_id]
 	verbose $result
-if {[lindex $result 2] == 0 && [lindex $result 3] == 23} {
-    pass "quit with expression"
-} else {
-    fail "quit with expression"
-}
-close $gdb_spawn_id
+	gdb_assert {[lindex $result 2] == 0 && [lindex $result 3] == 23} $test
+
+	remote_close host
 	clear_gdb_spawn_id
+    }
+}