ci/run-build-and-tests: add some structure to the GitHub workflow output

The current output of Git's GitHub workflow can be quite confusing,
especially for contributors new to the project.

To make it more helpful, let's introduce some collapsible grouping.
Initially, readers will see the high-level view of what actually
happened (did the build fail, or the test suite?). To drill down, the
respective group can be expanded.

Note: sadly, workflow output currently cannot contain any nested groups
(see https://github.com/actions/runner/issues/802 for details),
therefore we take pains to ensure to end any previous group before
starting a new one.

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

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