unit tests for PGLog::merge_old_entry

The tests covers 100% of the LOC of merge_old_entry. It is broken down
in 13 cases to enumerate all the situations it must address. Each case
is isolated in a independant code block where the conditions are
reproduced. For instance:

    info.last_backfill = hobject_t();
    info.last_backfill.hash = 1;
    oe.soid.hash = 2;

creates the conditions where merge_log_entry is expected to silently
ignore entries containing an object that is greater than
last_backfill.

PGLogTest is derived from PGLog to get access to the protected members.

Signed-off-by: Loic Dachary <loic@dachary.org>