[unit-tests/btree_counter_t] Add a couple of flushes().

This fixes the two failing btree_counter tests.  The blocknr in each
node is filled in when written out.  These days (thanks to Ming) the
counter also checks the btree, *but* if there are any problems
encoutered it just skips counting that node.

unit-tests/btree_counter_t.cc

@@ -61,6 +61,7 @@ namespace {
 TEST_F(BTreeCounterTests, count_empty_tree)
 {
 	tree_.reset(new btree<1, uint64_traits>(tm_, rc_));
+	tm_.get_bm()->flush();
 	check_nr_metadata_blocks_is_ge(1);
 }
 
@@ -73,6 +74,7 @@ TEST_F(BTreeCounterTests, count_populated_tree)
 		tree_->insert(key, 0ull);
 	}
 
+	tm_.get_bm()->flush();
 	check_nr_metadata_blocks_is_ge(40);
 }