Add default server restart to each test run

Found that the root cause of the issues happened with vinyl tests were
backside effects of the not correct test 'vinyl/gh.test.lua' which
leaved Tarantool worker process in inconsistent state. After it any
other next test on the same Tarantool worker process could fail on
running testings with snapshots calls, like tarantool/tarantool-qa#126:

  error: Snapshot is already in progress

Either restarting Tarantool worker process could fail on stopping it,
like #261 and tarantool/tarantool#5141:

  E> failed to process vylog record: delete_slice{slice_id=115, }
  E> ER_INVALID_VYLOG_FILE: Invalid VYLOG file: Slice 115 deleted but not registered

Decided to avoid of such situations for now and in the future all tests
must be run after Tarantool worker default server process was restarted.
This patch moves this server restart call from check if the test failed
to the common part of the tests run loop.

Part of tarantool/tarantool-qa#97
Fixes #260
Fixes #261
Needed for tarantool/tarantool#5089
Closes tarantool/tarantool-qa#126

Author: avtikhon

lib/worker.py

@@ -350,13 +350,13 @@ def run_loop(self, task_queue, result_queue):
             retries_left = self.suite.fragile_retries()
             # let's run till short_status became 'pass'
             while short_status != 'pass' and retries_left >= 0:
+                self.restart_server()
                 # print message only after some fails occurred
                 if short_status == 'fail':
-                    self.restart_server()
                     color_stdout(
                         'Test "%s", conf: "%s"\n'
                         '\tfrom "fragile" list failed with results'
-                        ' file checksum: "%s", rerunning with server restart ...\n'
+                        ' file checksum: "%s", rerunning ...\n'
                         % (task_id[0], task_id[1], result_checksum), schema='error')
                 # run task and save the result to short_status
                 short_status, result_checksum, duration = self.run_task(task_id)