test: switch to `sflight` and `bookshop` sub modules

[BobdenOs]

Tries to address the issues pointed out by https://github.com/cap-js/cds-dbs/pull/884 and https://github.com/cap-js/cds-dbs/pull/883

[danjoa]

As commented in #883 the approach is the right direction, but we should do so in some kind of umbrella setup like in cap/dev.