Expose bash scripts for local testing

[steve-chavez]

Problem

nix flake check runs all the tests we have, this can take a long time and the generated output files are big as well. This is inefficient, considering sometimes just one migration can be modified.

For these cases just running pg_regress over nix/tests would be enough. This should be much faster.

CI will still of course ensure nix flake check is run, but locally we can benefit from more speed.

Solution

Expose some tests as regular bash scripts, so developers can do:

$ nix develop

Then run:

$ <someprefix>-run-tests --kind migrations