test: enable parallel mode for wal_off tests

- Box configuration parameter `memtx_memory` is increased, because the
  test `lua` after `tuple` failed with the error:
  `Failed to allocate 368569 bytes in slab allocator for memtx_tuple`
  despite `collectgarbage('collect')` calls after cases with huge/many
  tuples.
  The statistics before the allocation fail gives the following values:
  ```
  box.slab.info()
  ---
  - items_size: 72786472
    items_used_ratio: 4.43%
    quota_size: 107374592
    quota_used_ratio: 93.75%
    arena_used_ratio: 6.1%
    items_used: 3222376
    quota_used: 100663296
    arena_size: 100663296
    arena_used: 6105960
  ```
  The reason of the fail seems to be a slab memory fragmentation. It is
  not clear for now whether we should consider this as a tarantool
  issue.

- Test `snapshot_stress` counts snapshot files present in the
  working directory and can reach the default 'checkpoint_count' value
  `2` if a previous test write its snapshots before.

- Restarting the default server w/o cleaning a working directory
  can leave a snapshot that holds a state saved at the middle of a test,
  before dropping of the space 'tweedledum' (because WAL is disabled),
  that can cause the error `Space 'tweedledum' already exists` for a
  following test.

- Use unix sockets because of errors `Address already in use`.

Part of #2436

(cherry picked from commit d837c94b)