Use matching single quotes in messages.

regress/open_nonarchive.test

diff --git a/regress/open_nonarchive.test b/regress/open_nonarchive.test
index c2a9fd5..c81b542 100644
--- a/regress/open_nonarchive.test
+++ b/regress/open_nonarchive.test
@@ -3,5 +3,5 @@
 file Makefile.am Makefile.am Makefile.am
 args Makefile.am
 return 1
-stdout opening `Makefile.am' returned error 19
+stdout opening 'Makefile.am' returned error 19
 stderr 1 errors