tests: properly position conditional output on Windows in test-subrepo.t

The test runner doesn't always guess the right location when optional output is
missing.  This goes with f6540aba8e3e.

Differential Revision: https://phab.mercurial-scm.org/D6640

tests/test-subrepo.t

@@ -315,8 +315,8 b' merge tests'
   resolving manifests
    branchmerge: True, force: False, partial: False
    ancestor: 6747d179aa9a, local: 20a0db6fbf6c+, remote: 7af322bc1198
+   preserving t for resolve of t
   starting 4 threads for background file closing (?)
-   preserving t for resolve of t
    t: versions differ -> m (premerge)
   picked tool ':merge' for t (binary False symlink False changedelete False)
   merging t