testsuite: Fix up strub-internal-pr112938.C test for C++2{0,3,6}

testsuite: Fix up strub-internal-pr112938.C test for C++2{0,3,6}

On Thu, Mar 27, 2025 at 12:05:21AM +0000, Sam James wrote:
> The test was being ignored because dg.exp looks for .C in g++.dg/.
>
> gcc/testsuite/ChangeLog:
>       PR middle-end/112938
>
>       * g++.dg/strub-internal-pr112938.cc: Move to...
>       * g++.dg/strub-internal-pr112938.C: ...here.

This regressed the test for C++20 and higher:
FAIL: g++.dg/strub-internal-pr112938.C  -std=gnu++20 (test for excess errors)
FAIL: g++.dg/strub-internal-pr112938.C  -std=gnu++23 (test for excess errors)
FAIL: g++.dg/strub-internal-pr112938.C  -std=gnu++26 (test for excess errors)

Here is a fix.

2025-03-27  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/strub-internal-pr112938.C: Add dg-warning for c++20.