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

@@ -1,33 +1,33 b''
   $ hg init outer
   $ cd outer
 
 hg debugsub with no remapping
 
   $ echo 'sub = http://example.net/libfoo' > .hgsub
   $ hg add .hgsub
 
   $ hg debugsub
   path sub
    source   http://example.net/libfoo
    revision 
 
 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
    source   C:\libs\foo-lib\
    revision 
 
 test bad subpaths pattern
 
   $ cat > .hg/hgrc <<EOF
   > [subpaths]
   > .* = \1
   > EOF
   $ hg debugsub
   abort: bad subrepository pattern in .*/test-subrepo-paths.t/outer/.hg/hgrc:2: invalid group reference
 
   $ exit 0