[TEST] Reduce forecast disk space requirement for tests (#34552)

The setting that reduces the disk space requirement
for the forecasting integration tests was accidentally
removed in #31757 when files were moved around.  This
change simply adds back the setting that existed before
that.

Author: David Roberts

x-pack/qa/ml-native-multi-node-tests/build.gradle

@@ -46,6 +46,7 @@ integTestCluster {
     setting 'xpack.security.transport.ssl.verification_mode', 'certificate'
     setting 'xpack.security.audit.enabled', 'true'
     setting 'xpack.license.self_generated.type', 'trial'
+    setting 'xpack.ml.min_disk_space_off_heap', '200mb'
 
     keystoreSetting 'bootstrap.password', 'x-pack-test-password'
     keystoreSetting 'xpack.security.transport.ssl.secure_key_passphrase', 'testnode'