raft_consensus-itest: fix waiting for log GC
raft_consensus-itest: fix waiting for log GC

The code that tried to wait for log GC was listing the children
of the WAL dir, but the actual WAL segments are inside a tablet
directory _within_ that WAL dir. This fixes the waiting code to
look in the correct place.

This fixes the remaining source of flakiness in the
TestFollowerFallsBehindLeaderGC test case. I looped it 1000 times
and it passed 100%.

Change-Id: I8db2aaa97f8d93b0c620a309533bb3079b8634ad
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/6697
Reviewed-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Reviewed-by: David Alves <33ea948168c114d220e0372a903be6ee60f6396e@cloudera.com>

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