Add JUnit support to MTR to generate XML test result

In MDEV-6039 there was a patch for this, but it was rejected.
Add JUnit reporting support to the MySQL test suite runner (Sergei Golubchik fb specific)
MySQL contains it https://github.com/mysql/mysql-server/blob/8.0/mysql-test/mysql-test-run.pl
the reasons to have that patch in the three are:
ci systems can draw charts based on test runs and show project health
test results may be seen locally by developers (variety of tools available)
JUnit format report is standard