tests: skip test-lfconvert.t if not support symblic link

Windows not support symbolic link. but test-lfconvert.t
execute 'ln -s' command.

@@ -51,8 +51,6 @@
   skipping incorrectly formatted tag IncorrectlyFormattedTag!
   skipping incorrectly formatted id invalidhash
   no mapping for id 0123456789abcdef
-  abort: renamed/copied largefile large3 becomes symlink
-  [255]
   $ cd bigfile-repo
   $ hg strip --no-backup 2
   0 files updated, 0 files merged, 2 files removed, 0 files unresolved

ERROR: test-lfconvert.t output changed

tests/test-lfconvert.t

@@ -1,3 +1,5 b''
+  $ "$TESTDIR/hghave" symlink || exit 80
+
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > largefiles =