Try dev version of Drush 9 with the 9.x Behat Drush Endpoint.

.circleci/config.yml

@@ -1,7 +1,7 @@
 # https://circleci.com/docs/2.0/workflows/#using-workspaces-to-share-data-among-jobs
 defaults: &defaults
   docker:
-    - image: quay.io/pantheon-public/build-tools-ci:1.x
+    - image: quay.io/pantheon-public/build-tools-ci:3.x
   working_directory: ~/example_drops_8_composer
   environment:
     #=========================================================================
@@ -39,7 +39,7 @@ defaults: &defaults
 version: 2
 jobs:
     # @todo: common initialization: 'composer install' for the site-under-test
-    build:
+    lint:
         <<: *defaults
         steps:
             - checkout
@@ -81,7 +81,7 @@ jobs:
                 name: run unit tests
                 command: composer -n unit-test
 
-    build_deploy_and_test:
+    functional:
         <<: *defaults
         steps:
             - checkout
@@ -132,9 +132,9 @@ jobs:
 
 workflows:
   version: 2
-  build_and_test:
+  test:
     jobs:
       # Install dev dependencies and do simple tests (sniff, unit tests, etc.)
-      - build
+      - lint
       # Build deploy and test on target platform
-      - build_deploy_and_test
+      - functional

.gitignore

@@ -4,6 +4,8 @@
 
 # Ignore directories generated by Composer
 /drush/contrib/
+/drush/Commands
+/drush/Commands/contrib/
 /vendor/
 /web/core/
 /web/modules/contrib/

composer.json

@@ -17,12 +17,12 @@
         "drupal/config_direct_save": "^1.0",
         "drupal/config_installer": "^1.0",
         "drupal/console": "^1",
+        "drush/drush": "8.x-dev",
         "drupal/core": "^8",
         "drupal/simple_block": "^1.0@beta",
-        "drush/drush": "~8",
-        "rvtraveller/qs-composer-installer": "^1.1",
-        "drush-ops/behat-drush-endpoint": "^0.0.5",
+        "drush-ops/behat-drush-endpoint": "dev-allow-8",
         "pantheon-systems/quicksilver-pushback": "~1",
+        "rvtraveller/qs-composer-installer": "^1.1",
         "webflo/drupal-core-strict": "^8"
     },
     "require-dev": {
@@ -40,7 +40,7 @@
     "conflict": {
             "drupal/drupal": "*"
     },
-    "minimum-stability": "alpha",
+    "minimum-stability": "dev",
     "prefer-stable": true,
     "autoload": {
         "classmap": [
@@ -77,7 +77,7 @@
             "web/modules/contrib/{$name}": ["type:drupal-module"],
             "web/profiles/contrib/{$name}": ["type:drupal-profile"],
             "web/themes/contrib/{$name}": ["type:drupal-theme"],
-            "drush/contrib/{$name}": ["type:drupal-drush"]
+            "drush/Commands/{$name}": ["type:drupal-drush"]
         },
         "build-env": {
             "install-cms": [