testsuite: g++.dg: rename pr42965 test

.c is used for C and .C is for C++. The test was being ignored before.

gcc/testsuite/ChangeLog:
	PR other/42965

	* g++.dg/warn/unused-result1-Werror.c: Move to...
	* g++.dg/warn/unused-result1-Werror.C: ...here.

diff --git a/gcc/testsuite/g++.dg/warn/unused-result1-Werror.c b/gcc/testsuite/g++.dg/warn/unused-result1-Werror.C
similarity index 100%
rename from gcc/testsuite/g++.dg/warn/unused-result1-Werror.c
rename to gcc/testsuite/g++.dg/warn/unused-result1-Werror.C