tests:w32: Fix for non-dot file name for Windows.

* tests/migrations/from-classic.scm (assert-migrated): Handle the case
on Windows.

Forward port from 2.2 branch of:
754175a46d3bc34e9ef8098dbd05abdfd61ada64

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>