New: Add classname attribute to JUnit testcase (refs #11068)

[hspazio]

What is the purpose of this pull request? (put an "X" next to item)
This PR is a follow up from the discussion in issue #11068.
With this change we are adding classname attribute to the <testcase> item.

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain: Add new attribute to JUnit formatter

What changes did you make? (Give an overview)

The value of classname is defined as the file path without extension.
Similar implementation to TSlint: palantir/tslint#4327

example:

<?xml version="1.0" encoding="utf-8"?>
<testsuites>
  <testsuite package="org.eslint" time="0" tests="1" errors="1" name="/path/to/foo.js">
    <testcase time="0" name="org.eslint.foo" classname="/path/to/foo">
      <failure message="Unexpected foo.">
        <![CDATA[line 5, col 10, Error - Unexpected foo. (foo)]]>
      </failure>
    </testcase>
  </testsuite>
</testsuites>

Is there anything you'd like reviewers to focus on?

Does this schema also need to be updated? https://eslint.org/docs/user-guide/formatters/#junit

[jsf-clabot]

All committers have signed the CLA.

[hspazio]

@not-an-aardvark @platinumazure Is there anything missing with this PR that I can help? Thank you.

[platinumazure]

LGTM, thanks! I would like another team member (or more than one) to review before merging.

[ilyavolodin]

Changes looks good to me. Can you rebase on the latest master please?

[hspazio]

Thank you @ilyavolodin. Rebased.

[dosuken123]

Awesome! @hspazio