ERT can generate JUnit test reports

ERT can generate JUnit test reports

* .gitignore: Add test/**/*.xml.

* admin/notes/emba: Mention JUnit test report.

* etc/NEWS: ERT can generate JUnit test reports.

* lisp/emacs-lisp/ert.el (xml-escape-string): Autoload.
(ert-write-junit-test-report)
(ert-write-junit-test-summary-report): New defuns.
(ert-run-tests-batch, ert-summarize-tests-batch-and-exit): Call them.

* test/Makefile.in (clean): Remove *.xml.

* test/README: Mention $EMACS_TEST_JUNIT_REPORT environment variable.

* test/infra/Makefile.in ($(FILE)): Generate header commentary.
(clean): Remove.

* test/infra/gitlab-ci.yml (variables): Set EMACS_TEST_JUNIT_REPORT.
(.job-template): Use it in script and after_script.
(.build-template, .gnustep-template, .filenotify-gio-template)
(.native-comp-template): Adapt rules.
(.test-template): Trigger JUnit test report.

* test/infra/test-jobs.yml: Regenerate.