testing: Use HA patch compatible with 4.15.6+

testing/testing.conf

@@ -28,7 +28,7 @@ fi
 : ${KERNEL=linux-$KERNELVERSION}
 : ${KERNELTARBALL=$KERNEL.tar.xz}
 : ${KERNELCONFIG=$DIR/../config/kernel/config-4.15}
-: ${KERNELPATCH=ha-4.15-abicompat.patch.bz2}
+: ${KERNELPATCH=ha-4.15.6-abicompat.patch.bz2}
 
 # strongSwan version used in tests
 : ${SWANVERSION=5.6.2}