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.