Add support for an integration and acceptance test suite using the
Subplot tool (https://subplot.liw.fi/). There are the initial, very
simple test scenarios, to get us started. The goal is to introduce the
scaffolding for integration tests, so that further tests can be added
with ease later.

The tests are documented and defined in sq-subplot.md. In build.rs, we
call Subplot to generate test code from the markdown file. The tests
are run via "cargo test", as usual.

Subplot can also generate a typeset test document from sq-subplot.md,
but we don't do that here.