Don't litter workdir when running tests

The tests are run in current workdir and call box.cfg which creates
snapshots and xlog files.

This patch keeps workdir clean by using the common approach:

- disable xlog using `wal_mode = 'none'`
- create snapshot in a temp dir and remove it immediately

Author: rosik

tests/run_tests.lua

@@ -1,11 +1,18 @@
 #!/usr/bin/env tarantool
 
-fiber = require('fiber')
+local fio = require('fio')
+local fiber = require('fiber')
+
+local tmpdir = fio.tempdir()
 
 box.cfg{
     listen = 3301,
+    wal_mode = 'none',
+    memtx_dir = tmpdir,
 }
 
+fio.rmtree(tmpdir)
+
 -- Init test database
 box.once('bootstrap_tests', function()
     box.schema.user.create('test_user', { password = 'password' })