Maybe this one fixes the test?

test/test-helper.el

@@ -53,15 +53,15 @@ Finally, the indented text in the buffer will be compared with AFTER."
   (with-temp-buffer
     ;; fix the JAR location prior to mode initialization
     ;; for some reason, plantuml-mode disregards the setq-local
-    (setq-local plantuml-jar-path plantuml-test-jar-path)
+    (setq plantuml-jar-path plantuml-test-jar-path)
     (plantuml-init-once)
 
     (insert before)
     (goto-char (point-min))
     (plantuml-mode)
     ;; use 2 spaces instead of one tab for indentation
-    (setq-local indent-tabs-mode nil)
-    (setq-local tab-width 2)
+    (setq indent-tabs-mode nil)
+    (setq tab-width 2)
 
     (indent-region (point-min) (point-max))
     (should (equal (buffer-string) after))))