Make dprintf-non-stop.exp cope with remote testing

Testing with the extended-remote board with "maint set target-non-stop
on" shows a dprintf-non-stop.exp regression.  The issue is simply that
the test is expecting output that is only valid for the native target:

 native:

  [process 8676] #1 stopped.

 remote:

  [Thread 8900.8900] #1 stopped.

In order to expose this without "maint set target-non-stop on", this
restarts gdb with non-stop mode already enabled.

gdb/testsuite/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* gdb.base/dprintf-non-stop.exp: Use build_executable instead of
	prepare_for_testing.  Start gdb with "set non-stop on" appended to
	GDBFLAGS.  Lax expected stop output.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-11-30  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/dprintf-non-stop.exp: Use build_executable instead of
+	prepare_for_testing.  Start gdb with "set non-stop on" appended to
+	GDBFLAGS.  Lax expected stop output.
+
 2015-11-27  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.arch/arm-neon.exp: New.

gdb/testsuite/gdb.base/dprintf-non-stop.exp

@@ -20,13 +20,17 @@ if [is_remote target] then {
 }
 
 standard_testfile
+set executable ${testfile}
 
-if [prepare_for_testing "failed to prepare for dprintf with non-stop" \
+if [build_executable "failed to prepare for dprintf with non-stop" \
     ${testfile} ${srcfile} {debug}] {
     return -1
 }
 
-gdb_test_no_output "set non-stop on"
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " -ex \"set non-stop on\""
+    clean_restart ${executable}
+}
 
 if ![runto main] {
     fail "Can't run to main"
@@ -60,7 +64,7 @@ gdb_test_multiple $test $test {
 
 set test "inferior stopped"
 gdb_test_multiple "" $test {
-    -re "\r\n\\\[.* \[0-9\]+\\\] #1 stopped\\\.\r\n" {
+    -re "\r\n\\\[.*\\\] #1 stopped\\\.\r\n" {
 	pass $test
     }
 }