Add truncate space on cluster in tests for stats

Before this commit `simple_operation_cases`
were organized as map
(table indexed not with numbers),
in Lua iteration over map does not occur in
the order in which the elements were specified
in the map.
But simple operation tests could fail
in case if tests would be executed not
in the order in which they are specified,
because, for example, if `replace()` is performed
before `insert()`, an error will be received.
So simple operation tests are codependent.
To solve this problem `truncate_space_on_cluster`
was added after each simple operation test.

Part of #193

test/integration/stats_test.lua

@@ -119,6 +119,10 @@ local function create_new_space(g)
     end)
 end
 
+local function truncate_space_on_cluster(g)
+    helpers.truncate_space_on_cluster(g.cluster, space_name)
+end
+
 -- If there weren't any operations, space stats is {}.
 -- To compute stats diff, this helper return real stats
 -- if they're already present or default stats if
@@ -446,6 +450,8 @@ local function generate_stats(g)
         else
             t.assert_not_equals(err, nil)
         end
+
+        helpers.truncate_space_on_cluster(g.cluster, space_name)
     end
 
     -- Generate non-null select details.
@@ -559,6 +565,8 @@ for name, case in pairs(simple_operation_cases) do
 
         t.assert_equals(unchanged_before, unchanged_after, 'Other stats remained the same')
     end
+
+    pgroup.after_test(test_name, truncate_space_on_cluster)
 end