Add IntegrationTest::app_dir_preprocessor

[Malax]

This allows users to have the app directory (test fixture) to be modified in a safe way, just before the
integration test runs.

Usage Example

This is a cutlass test ported to libcnb-test for heroku/maven. Original cutlass test can be found here: https://github.com/heroku/buildpacks-jvm/blob/e91747a4310d566a2ecf3b88ce94ed86744c6346/test/specs/maven/misc_spec.rb#L3-L13

#[test]
#[cfg(target_family = "unix")]
pub fn maven_wrapper_executable_bit() {
    IntegrationTest::new(BUILDER_NAME, "../../test/fixtures/simple-http-service")
        .buildpacks(vec![
            BuildpackReference::Other(String::from("heroku/jvm")),
            BuildpackReference::Crate,
        ])
        .app_dir_preprocessor(|app_dir| {
            use std::os::unix::fs::PermissionsExt;
            fs::set_permissions(app_dir.join("mvnw"), Permissions::from_mode(0o444)).unwrap();
        })
        .run_test(|context| {
            assert_contains!(context.pack_stdout, "Successfully built image");
        });
}

Closes GUS-W-11177468

[edmorley]

In the future, perhaps we can switch to only performing the app_dir copy iff app_dir_preprocessor is set? We still expose app_dir during the test itself, and in the rustdocs for it say modifying is fine - but perhaps when we eventually support repeat builds, we could make users use app_dir_preprocessor for repeat builds too, at which point we can make app_dir non-public on IntegrationTestContext, and so it would be safe to not copy unless a pre-processor is set?

[Malax]

In the future, perhaps we can switch to only performing the app_dir copy iff app_dir_preprocessor is set? We still expose app_dir during the test itself, and in the rustdocs for it say modifying is fine - but perhaps when we eventually support repeat builds, we could make users use app_dir_preprocessor for repeat builds too, at which point we can make app_dir non-public on IntegrationTestContext, and so it would be safe to not copy unless a pre-processor is set?

In fact, this is broken out of a larger change that allows repeat builds. The idea is to allow app_dir_preprocessor for repeat builds as well. 👍🏻 It is still under heavy development, I'll consider removing app_dir from IntegrationTestContext and dropping app_dir copying when it's not required.

I assume your main concern is removing unnecessary IO during tests (the app_dir copy) for a slight speed-up?

[edmorley]

I assume your main concern is removing unnecessary IO during tests (the app_dir copy) for a slight speed-up?

Yeah