CircleCI: Add PHP 7.4 test

Author: tangrufus

.circleci/config.yml

@@ -16,6 +16,13 @@ workflows:
             - codecov/upload:
                 file: tests/_output/coverage.xml
 
+      - composer/install:
+          name: test-7.4
+          executor: composer/php-74
+          post-steps:
+            - composer/exec:
+                command: test
+
       - composer/install:
           name: test-7.3
           executor: composer/php-73