Provide tests/README.md for usage instructions, when trying to reproduce
the GitHub tests inside a local VM.

Our tests/build-vm-and-test.sh script relies on presence of curl
command, so ensure it's available.

We need to replace `/usr/bin/env python` with `/usr/bin/env python3`
as /usr/bin/python is provided only by the python-is-python3 or
python-is-python2 packages through a symlink (meant for third-party or
legacy scripts, see https://wiki.debian.org/Python), though those might
not be present e.g. when running in a local Debian bookworm VM.

For bookworm.

… fails

While debugging the failing GitHub Actions (see #278), we noticed that
it might serve useful to have a screenshot of the booted system
available. We can capture such a screenshot via vncsnapshot.

NOTE: we move the files inside `results` only if the directory as such
exists in $PWD, otherwise we'd fail the build when being executed as
`tests/build-vm-and-test.sh test` from inside the grml-debootstrap.git.
This happens when manually reproducing the GH tests, though then
`build-vm-and-test.sh` is supposed to be executed from the parent
directory of the `tests` directory (being the grml-debootstrap.git
checkout).