fix: Try using a test task specific folder for user preference to avoid locks

Author: bric3

buildSrc/src/main/kotlin/dd-trace-java.configure-tests.gradle.kts

@@ -42,6 +42,10 @@ tasks.withType<Test>().configureEach {
     !rootProject.providers.gradleProperty("rerun.tests.${project.name}").isPresent
   }
 
+  // Trick to avoid on CI: "Couldn't flush user prefs: java.util.prefs.BackingStoreException: Couldn't get file lock."
+  // Use a task-specific user prefs directory
+  systemProperty("java.util.prefs.userRoot", "$buildDir/tmp/userPrefs/${name}")
+
   // Split up tests that want to run forked in their own separate JVM for generated tasks
   if (name.startsWith("forkedTest") || name.endsWith("ForkedTest")) {
     setExcludes(emptyList())