Write testcase for FSLeafQueue that explicitly checks if non-zero AM-share values are not overwritten for custom resources

This is a follow-up for YARN-9323, covering changes regarding explicit zero value check that has been discussed with templedf earlier.
YARN-9323 fixed a bug in FSLeafQueue#computeMaxAMResource, so that custom resource values are also set to the AM share.
We need a new test in TestFSLeafQueue that explicitly checks if the custom resource value is only being set if the fairshare for that resource is zero.
This way, we can make sure we don't overwrite any meaningful resource value.