Because Elasticsearch treats those numbers as Longs by default, you need to make sure that you pass a Long to the contains method. The following should work:
GET testdatatype_unit_tests/_search
{
"size": 100,
"query": {
"script": {
"script": {
"source": "doc['IntCollection'].values.contains(1L)",
"lang": "painless"
}
}
}
}