test: fix flaky gh_6539_log_user_space_empty_or_nil_select_test

The test fails with:

  not ok 2 ...ty_or_nil_select.test_log_entry_presence_for_memtx_user_space
  gh_6539_log_user_space_empty_or_nil_select_test.lua:122: log must contain
  a critical entry about `box.space.test_memtx:select({0}, {limit = 1001,
  iterator = "ALL"})` call on a memtx user space
  expected: a value evaluating to true, actual: nil

Looks like it happens, because g.server:grep_log() is executed before
g.server:eval(call) completes writing to the log.
In other tests grep_log() is wrapped into t.helpers.retrying() in order
to avoid such issues, so do the same here.

May close tarantool/tarantool-qa#264

NO_DOC=test fix
NO_CHANGELOG=test fix