Aggregate testgroups output in a junit output

Reported by @ndufresne

For inspiration:

• https://gitlab.collabora.com/linux/build-scripts/-/blob/master/ci/summary_to_junit.py
• https://gitlab.collabora.com/linux/build-scripts/-/blob/master/ci/ci-run-tests.sh?ref_type=heads#L33

Ideally, on the testgroup we could do something like:

sh "$CI_DIR"/aggregate-junit.sh sh "$CI_DIR"/test-build.sh --arch x86_64 --cross_compile x86_64-linux-gnu-
sh "$CI_DIR"/aggregate-junit.sh sh "$CI_DIR"/test-build.sh --arch x86 --cross_compile i686-linux-gnu-