[Fix #9780] Support summary report for JUnitFormatter

[koic]

Fixes #9780.

This PR supports summary report for JUnitFormatter.

It adds the following attributes to the <testsuite> element.

• tests attribute ... Inspected file count.
• failures attribute ... Total offense count.

This is a JUnit format example:

% rubocop -f junit --display-only-failed lib/rubocop/formatter/junit_formatter.rb
<?xml version='1.0'?>
<testsuites>
  <testsuite name='rubocop' tests='1' failures='2'>
    <testcase classname='lib.rubocop.formatter.junit_formatter' name='Layout/LineLength'>
      <failure type='Layout/LineLength' message='Layout/LineLength: Line is too long. [110/100]'>
        /Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/formatter/junit_formatter.rb:65:101
      </failure>
    </testcase>
    <testcase classname='lib.rubocop.formatter.junit_formatter' name='Metrics/AbcSize'>
      <failure type='Metrics/AbcSize' message='Metrics/AbcSize:
Assignment Branch Condition size for file_finished is too high. [&lt;8, 16, 2&gt; 18/17]'>
        /Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/formatter/junit_formatter.rb:29:7
      </failure>
    </testcase>
  </testsuite>
</testsuites>

These mean the values of 1 file inspected and 2 offenses detected below.

% rubocop lib/rubocop/formatter/junit_formatter.rb
Inspecting 1 file
(snip)

1 file inspected, 2 offenses detected, 2 offenses auto-correctable

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!