atf, lutok, kyua: compile with CXXSTD=gnu++20

All three of the test infrastructure components require at least C++20
in order to build. Make that version the default to better match what
is required in recent versions of the projects.

MFC after:	1 week

lib/atf/common.mk

@@ -16,3 +16,5 @@ atf-version-real: .PHONY
 	@grep 'define VERSION' ${ATF}/config.h \
 	    | cut -d '"' -f 2 >atf-version-real
 CLEANFILES+= atf-version atf-version-real
+
+CXXSTD=		gnu++20

lib/liblutok/Makefile

@@ -18,4 +18,6 @@ CFLAGS+=	-I${SRCTOP}/contrib/lutok/include \
 WARNS?=	3
 SHLIB_MAJOR=	0
 
+CXXSTD=		gnu++20
+
 .include <bsd.lib.mk>