Fix flaky hardlinks test

The test was added in dc34990, it turns out that it's flaky. It failed
once on the Debian build infra, cf. [1].

The problem is that the command `rsync -aH '$fromdir/sym' '$todir'`
updates the mod time of `$todir`, so there might be a diff between the
output of `rsync_ls_lR $fromdir` and `rsync_ls_lR $todir`, if ever rsync
runs 1 second (or more) after the directories were created.

To clarify: it's easy to make the test fails 100% of the times with this
change:

```
 makepath "$fromdir/sym" "$todir"
+sleep 5
 checkit "$RSYNC -aH '$fromdir/sym' '$todir'" "$fromdir" "$todir"
```

With the fix proposed here, we don't use `checkit` anymore, instead we
just run the rsync command, then a simple `diff` to compare the two
directories. This is exactly what the other `-H` test just above does.

In case there's some doubts, `diff` fails if `sym` is missing:

```
$ mkdir -p foo/sym bar
$ diff foo bar || echo KO!
Only in foo: sym
KO!
```

I tested that, after this commit, the test still catches the `-H`
regression in rsync 3.4.0.

Fixes: #735

[1]: https://buildd.debian.org/status/fetch.php?pkg=rsync&arch=ppc64el&ver=3.4.1%2Bds1-1&stamp=1741147156&raw=0

Author: elboulangero

testsuite/hardlinks.test

@@ -81,7 +81,8 @@ diff $diffopt "$name1" "$todir" || test_fail "solo copy of name1 failed"
 # enabled (this has broken in 3.4.0 so far, so we need this test).
 rm -rf "$fromdir" "$todir"
 makepath "$fromdir/sym" "$todir"
-checkit "$RSYNC -aH '$fromdir/sym' '$todir'" "$fromdir" "$todir"
+$RSYNC -aH "$fromdir/sym" "$todir"
+diff $diffopt "$fromdir" "$todir" || test_fail "solo copy of sym failed"
 
 # The script would have aborted on error, so getting here means we've won.
 exit 0