After https://github.com/matrix-org/synapse/pull/8377, the deb packages no longer indirectly installed the `"test"` dependencies, causing debian packages to fail to build while carrying out the unit tests.

This PR installs `test` dependencies explicitly when building debian packages.

This ensures systemctl start matrix-synapse returns only after synapse
is actually started, which is very useful for automated deployments.

Fixes #5761

Signed-off-by: Dexter Chua <dec41@srcf.net>

Since this is not a full release yet