test-subrepo-paths: use printf instead of echo

Different shells have different opinions on how to backslashes must be
escaped for echo, but printf is consistent across zsh, dash, bash,
tcsh, and csh.

tests/test-subrepo-paths.t

@@ -14,7 +14,7 b' hg debugsub with no remapping'
 hg debugsub with remapping
 
   $ echo '[subpaths]' > .hg/hgrc
-  $ echo 'http://example.net/lib(.*) = C:\libs\\1-lib\' >> .hg/hgrc
+  $ printf 'http://example.net/lib(.*) = C:\\libs\\\\1-lib\\\n' >> .hg/hgrc
 
   $ hg debugsub
   path sub