Enable the poetry_cache_previous_buildpack_version test (#263)

This test checks compatibility of cached Poetry layers from previous
versions of the buildpack. However, until there was at least one previous
buildpack release that supported Poetry, the test had to be skipped.

Now that v0.17.0 of the buildpack has been released (which includes
Poetry support), we can enable this test.

tests/poetry_test.rs

@@ -154,13 +154,9 @@ fn poetry_cache_invalidation_package_manager_changed() {
 #[test]
 #[ignore = "integration test"]
 fn poetry_cache_previous_buildpack_version() {
-    #![allow(unreachable_code)]
-    // TODO: Enable this test once a previous buildpack release exists that supports Poetry.
-    return;
-
     let mut config = default_build_config("tests/fixtures/poetry_basic");
     config.buildpacks([BuildpackReference::Other(
-        "docker://docker.io/heroku/buildpack-python:TODO".to_string(),
+        "docker://docker.io/heroku/buildpack-python:0.17.0".to_string(),
     )]);
     let rebuild_config = default_build_config("tests/fixtures/poetry_basic");
 
@@ -170,8 +166,23 @@ fn poetry_cache_previous_buildpack_version() {
             assert_contains!(
                 rebuild_context.pack_stdout,
                 &formatdoc! {"
-                        TODO
-                    "}
+                    [Determining Python version]
+                    No Python version specified, using the current default of Python {DEFAULT_PYTHON_VERSION}.
+                    To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes
+                    
+                    [Installing Python]
+                    Using cached Python {DEFAULT_PYTHON_VERSION}
+                    
+                    [Installing Poetry]
+                    Using cached Poetry {POETRY_VERSION}
+                    
+                    [Installing dependencies using Poetry]
+                    Using cached virtual environment
+                    Running 'poetry install --sync --only main'
+                    Installing dependencies from lock file
+                    
+                    No dependencies to install or update
+                "}
             );
         });
     });