Try to reconcile magento/mtf updates and PHPUnit updates.

dev/tests/functional/.gitignore

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+vendor

dev/tests/functional/README.md

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+# Magento Functional Tests
+
+This suite of tests is based on the abandoned magento/mtf project.
+
+## Install
+
+First, install dependencies using composer:
+
+```sh
+docker run --rm -it \
+  --volume $PWD:/app --volume ${COMPOSER_HOME:-$HOME/.composer}:/tmp \
+  --user $(id -u):$(id -g) \
+  composer --ignore-platform-reqs install
+```
+
+Run tests:
+
+```sh
+docker run --rm -it \
+  --volume $(realpath $PWD/../../../):/app --workdir /app \
+  php:7.2 \
+    dev/tests/functional/vendor/phpunit/phpunit/phpunit -c dev/tests/functional \
+    dev/tests/functional/testsuites/Magento/Mtf/TestSuite/InjectableTests.php
+```