test: add integration tests

[tinybeachthor]

Add integration tests to detect regressions for #221, #235.
Run tests in github action.

[DavHau]

Wonderful! Could you add a comment to the top of each flake.nix explaining in one or two sentences why the test exists / what it does?

[yusdacra]

This is great, but doesn't this basically copy all of examples testing code? Maybe instead of copying it all we could parametrize the testing code so we can pass examples dir and integration-tests dir etc.

[tinybeachthor]

@DavHau
Added descriptions for the integration tests

@yusdacra
The test runner is not the same, for examples we are running nix check and don't evaluate haskell packages, for the integration tests we want to actually build the default package.
I personally don't mind code duplication in test setups. Unifying and parameterizing the scripts would only subtract from readability.