tests: add hexquote_strndup function to libtests

* tests/tests.h (hexquote_strndup): New prototype.
* tests/hexquote_strndup.c: New file.
* tests/Makefile.am (libtests_a_SOURCES): Add it.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 667d863..da74adc 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -44,6 +44,7 @@
 	error_msg.c \
 	get_page_size.c \
 	hexdump_strdup.c \
+	hexquote_strndup.c \
 	inode_of_sockfd.c \
 	tail_alloc.c \
 	tests.h \