This is useful as someone downstream can source the
`scripts-dev/complement.sh` script and run the same set of tests as
Synapse:
```bash
# Grab the test packages supported by Synapse.
#
# --fast: Skip rebuilding the docker images,
# --build-only: Will only build Docker images but because we also used `--fast`, it won't do anything.
# `>/dev/null` to redirect stdout to `/dev/null` to get rid of the `echo` logs from the script.
test_packages=$(source ${SYNAPSE_DIR}/scripts-dev/complement.sh --fast --build-only >/dev/null && echo "$SYNAPSE_SUPPORTED_COMPLEMENT_TEST_PACKAGES")
echo $test_packages
```
This is spawning from wanting to run the same set of Complement tests in
the https://github.com/element-hq/synapse-rust-apps project.
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