tests: Change the time function used in the bookmark tests

The current comparison has been seen to fail on vs2017-x86
(https://gitlab.gnome.org/pwithnall/glib/-/jobs/2643197):
```
not ok /bookmarks/deprecated - GLib:ERROR:../glib/tests/bookmarkfile.c:921:test_deprecated: assertion failed (t >= now): (1678122080 >= 1678122081)
```

I guess this is caused by a mismatch between the system clock as used by
`time (NULL)` and `g_get_real_time ()` (which is ultimately what
`g_bookmark_file_set_added()` uses) on Windows.

Attempt to fix that by using `g_get_real_time()` in the test harness
too, so the clock being used is consistent.

If that doesn’t work, my next guess is that some of the code in
`test_deprecated()` is being reordered by the compiler. But that
shouldn’t happen, because the `time()` call has side-effects, and
`g_bookmark_file_set_added()` has side-effects, so they shouldn’t be
reordered with respect to each other. And certainly not so that the
latter one (in code order) ends up being called 1s after the other.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

glib/tests/bookmarkfile.c

@@ -911,7 +911,7 @@ test_deprecated (void)
 
 G_GNUC_BEGIN_IGNORE_DEPRECATIONS
 
-  now = time (NULL);
+  now = g_get_real_time () / G_USEC_PER_SEC;
   file = g_bookmark_file_new ();
 
   /* added */