maint: prohibit translations in testsuite

Nothing in the testsuite or examples directory should be translated,
as it is not part of the normally installed binary.  We already
meet this rule, but enforcing it will make it easier to remember.

Suggested by Daniel P. Berrange.

* cfg.mk (sc_prohibit_useless_translation): Enhance rule.

cfg.mk

@@ -587,9 +587,14 @@ sc_prohibit_diagnostic_without_format:
 	    exit 1; } || :
 
 # The strings "" and "%s" should never be marked for translation.
+# Files under tests/ and examples/ should not be translated.
 sc_prohibit_useless_translation:
 	@prohibit='_\("(%s)?"\)'					\
-	halt='$(ME): found useless translation'				\
+	halt='found useless translation'				\
+	  $(_sc_search_regexp)
+	@prohibit='\<N?_ *\('						\
+	in_vc_files='^(tests|examples)/'				\
+	halt='no translations in tests or examples'			\
 	  $(_sc_search_regexp)
 
 # Enforce recommended preprocessor indentation style.