Make tests detect UTF-8 locale with a helper executable

This uses the same utility function that mosh-client/mosh-server do.
This resolves portability issues with the 'locale' command.
This fixes OpenBSD 6.0 and probably Haiku builds.

src/tests/Makefile.am

@@ -35,7 +35,7 @@ displaytests = \
 	unicode-later-combining.test \
 	window-resize.test
 
-check_PROGRAMS = ocb-aes encrypt-decrypt base64 nonce-incr inpty
+check_PROGRAMS = ocb-aes encrypt-decrypt base64 nonce-incr inpty is-utf8-locale
 TESTS = ocb-aes encrypt-decrypt base64 nonce-incr local.test $(displaytests)
 XFAIL_TESTS = \
 	e2e-failure.test \
@@ -65,6 +65,10 @@ inpty_SOURCES = inpty.cc
 inpty_CPPFLAGS = -I$(srcdir)/../util
 inpty_LDADD = ../util/libmoshutil.a $(LIBUTIL)
 
+is_utf8_locale_SOURCES = is-utf8-locale.cc
+is_utf8_locale_CPPFLAGS = -I$(srcdir)/../util
+is_utf8_locale_LDADD = ../util/libmoshutil.a $(LIBUTIL)
+
 clean-local: clean-local-check
 .PHONY: clean-local-check
 clean-local-check: