Disable parallel tests on Gradle again

Something is causing parallel tests to fail on CI. I haven't been able to track down what it is. The symptoms look similar to others where the cause was a test modifying the test-data directory.

The integration tests also sometimes run into OutOfMemoryErrors when I run them in parallel.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1887631 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/build.gradle b/build.gradle
index 23232af..cd7d8a4 100644
--- a/build.gradle
+++ b/build.gradle
@@ -167,7 +167,7 @@
         // this is necessary for JDK 9+ to keep formatting dates the same way as in previous JDK-versions
         systemProperties['java.locale.providers'] = 'JRE,CLDR'
 
-        systemProperties['junit.jupiter.execution.parallel.enabled'] = 'true'
+        systemProperties['junit.jupiter.execution.parallel.enabled'] = 'false'
     }
 
     jacoco {