c++: adjust some tests for modules

We aren't enabling modules by default yet, but let's fix these tests now so
they won't fail when that happens.

gcc/testsuite/ChangeLog:

	* g++.dg/template/error25.C: Adjust export diagnostic.
	* g++.old-deja/g++.benjamin/tem05.C: Likewise.
	* g++.old-deja/g++.pt/export1.C: Likewise.
	* g++.dg/pch/pch.exp: Specify -fno-modules.