Add Minitest::Assertion to the error tolerance list of #patiently

I'm using spreewald #patiently method.
But it doesn't work well with minitest.
If we just put Ministest::Assertion to ToleranceForSeleniumSyncIssues::RETRY_ERRORS makes it work well.
Meanwhile I'm using my branch with
```ruby
group :test do
  # ...
  gem 'spreewald', :require => false, github: "abinoam/spreewald", branch: "feat_add_minitest_to_silently"
  gem 'minitest'
  # ...
end
```

I'm opening this pull request because perhaps you're interested in making spreewald #silently method to work with minitest.