tests: test-check-code-hg.t works for all files to check

with xargs, backslashes are eaten up. Convert them to slashes therefore.
This is only a problem with ls (on windows). hg manifest returns slashes.

The pipe char is moved before the line end for telling check-code.py that sed
does not modify the output.

tests/test-check-code-hg.t

@@ -26,6 +26,5 b' Prepare check for Python files without p'
 
 New errors are not allowed. Warnings are strongly discouraged.
 
-  $ { hg manifest 2>/dev/null; ls "$TESTTMP"/*.py; } \
-  >   | xargs "$check_code" --warnings --per-file=0 \
-  >   || false
+  $ { hg manifest 2>/dev/null; ls "$TESTTMP"/*.py | sed 's-\\-/-g'; } |
+  >   xargs "$check_code" --warnings --per-file=0 || false