src/test/osd/TestPGLog.cc: check that the object remains in log.objects

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
PGLog::rewind_divergent_log: unindex only works from tail, index() instead

Fixes: #5714
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
PGLog::merge_log, unidex() only works from tail, we index() below anyway

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>