Add support for JUnit legacy XML reports

[romain-gilles-ultra]

Some CI environments provide Unit test reports but for that, they need a specific output format.

This PR adds a general usage module to export TestResult into JUnit legacy XML test suite reports.

[lefou]

@romain-gilles-ultra I just merged #3099, which provides almost the same report (I guess) out of the box. Can you please comment, if this PR is now obsolete or still valid?

[romain-gilles-ultra]

Hi @lefou
You can close this PR I will check if it works for me when I get a new night build 🤗
But when I checked this documentation https://github.com/testmoapp/junitxml and the code of sbt there are a couple of differences for example there is no concept of id in a testsuite and testcase. In my PR I was doing so grouping too.
If it does not work for me on gitlab I will create an new PR.