As a small addendum to the last post, here are the relevant commands #debci helpfully provided.

First, you need to install the autopkgtest package, obviously:

# apt install autopkgtest

Then you need to create a Debian virtual machine to run the tests (put the sid.raw wherever you prefer):

# autopkgtest-build-qemu sid /tmp/sid.raw

Then you can run the tests themselves, using the just created virtual machine. The autopkgtest command can use the tests from various sources, using the last argument to the command. In my case what was the most helpful was to run the tests from my git clone (which uses gbp) so I could edit the tests directly. So I didn't give anything for testsrc (but . would work as well I guess).

$ autopkgtest -BU --add-apt-release=unstable --pin-packages=unstable=strongswan -- qemu /tmp/sid.raw --ram-size=4096 --cpus=1

Then I could play with the tests themselves, the number of CPU for the Qemu VM, and run everything in a loop.