tests: refactor --write-junit-xml code

tests: refactor --write-junit-xml code

The code writing JUnit XML is interspersed directly with all the code in
`t/test-lib.sh`, and it is therefore not only ill-separated, but
introducing yet another output format would make the situation even
worse.

Let's introduce an abstraction layer by hiding the JUnit XML code behind
four new functions that are supposed to be called before and after each
test and test case.

This is not just an academic exercise, refactoring for refactoring's
sake. We _actually_ want to introduce such a new output format, to
make it substantially easier to diagnose test failures in our GitHub
workflow, therefore we do need this refactoring.

This commit is best viewed with `git show --color-moved
--color-moved-ws=allow-indentation-change <commit>`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>