Data race in TestConnection_NewWatcher_concurrent

[better0fdead]

Have faced a data race in github ci run.

==================
WARNING: DATA RACE
Write at 0x00c00005e750 by goroutine 91:
  github.com/tarantool/go-tarantool_test.TestConnection_NewWatcher_concurrent.func1()
      /Users/runner/work/go-tarantool/go-tarantool/tarantool/go-tarantool/tarantool_test.go:3854 +0x377

Previous write at 0x00c00005e750 by goroutine 203:
  github.com/tarantool/go-tarantool_test.TestConnection_NewWatcher_concurrent.func1()
      /Users/runner/work/go-tarantool/go-tarantool/tarantool/go-tarantool/tarantool_test.go:3854 +0x377

Goroutine 91 (running) created at:
  github.com/tarantool/go-tarantool_test.TestConnection_NewWatcher_concurrent()
      /Users/runner/work/go-tarantool/go-tarantool/tarantool/go-tarantool/tarantool_test.go:3840 +0x435
  testing.tRunner()
      /Users/runner/hostedtoolcache/go/1.13.15/x64/src/testing/testing.go:909 +0x199

Goroutine 203 (running) created at:
  github.com/tarantool/go-tarantool_test.TestConnection_NewWatcher_concurrent()
      /Users/runner/work/go-tarantool/go-tarantool/tarantool/go-tarantool/tarantool_test.go:3840 +0x435
  testing.tRunner()
      /Users/runner/hostedtoolcache/go/1.13.15/x64/src/testing/testing.go:909 +0x199
==================
--- FAIL: TestConnection_NewWatcher_concurrent (1.49s)
    tarantool_test.go:3863: An error found: Unable to get an event 49
    testing.go:853: race detected during execution of test