Maintenance: provide expected failure metadata in junit reports

[yannbf]

Issue: N/A

What I did

• Fixed a bug where junit.xml files were getting overwritten when they shouldn't
• The e2e tests will now attach metadata in case we use test.fail to Junit reports, for expected failures:

<!-- Example with an expected failure -->
<testsuite name="addon-interactions.spec.ts" timestamp="1667901141467" hostname="" tests="1" failures="0" skipped="0" time="6.584" errors="0">
  <testcase name="addon-interactions should have interactions" classname="[chromium] › addon-interactions.spec.ts › addon-interactions › should have interactions" time="6.584">
    <properties>
      <property name="fail" value="React Vite (JS) is failing, however it is expected">
      </property>
    </properties>
  </testcase>
</testsuite>

<!-- Example with no expected failure -->
<testsuite name="addon-viewport.spec.ts" timestamp="1667901141467" hostname="" tests="1" failures="0" skipped="0" time="1.724" errors="0">
  <testcase name="addon-viewport should have viewport button in the toolbar" classname="[chromium] › addon-viewport.spec.ts › addon-viewport › should have viewport button in the toolbar" time="1.724">
  </testcase>
</testsuite>

This way, if we have an e2e test that we know is failing, but we don't want the CI to be red all the time, however we want to know that failure so we can process it somewhere else, we can.

Here's an example of test.fail usage:

test('should have interactions', async ({ page }) => {
    test.fail(
      // eslint-disable-next-line jest/valid-title
      /^(Lit)/i.test(`${templateName}`),
      `${templateName} is failing, however it is expected`
    );
});

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.