coroutines, testsuite: Require C++17 for two tests.

While the coroutines implementation, and most of the coroutines
tests, will operate with C++14 or newer, these tests require
facilities introduced in C++17.  Add the target requirement.

gcc/testsuite/

2020-04-19  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/co-await-17-capture-comp-ref.C: Require
	C++17.
	* g++.dg/coroutines/torture/co-ret-15-default-return_void.C: Likewise.

gcc/testsuite/ChangeLog

+2020-04-19  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* g++.dg/coroutines/torture/co-await-17-capture-comp-ref.C: Require
+	C++17.
+	* g++.dg/coroutines/torture/co-ret-15-default-return_void.C: Likewise.
+
 2020-04-19  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/94347

gcc/testsuite/g++.dg/coroutines/torture/co-await-17-capture-comp-ref.C

-//  { dg-do run }
+//  { dg-do run { target c++17 } }
 
 #include "../coro.h"
 

gcc/testsuite/g++.dg/coroutines/torture/co-ret-15-default-return_void.C

-// { dg-do run }
+// { dg-do run { target c++17 } }
 //
 // Check if default return_void is insert at correct position.
 #include <cassert>