Add more integration tests for painless extensions.

[mashhurs]

Description

Recently we have identified an issue with painless process that if it has a reference to Processors#method() plugin crashed. We have identified the root cause and added painless extension modules (PR: #162). We added single integration test to speed up the delivery and we need to add more integration tests for each extension.

extensions.add(new ConstantKeywordPainlessExtension()); // module: constant-keyword
extensions.add(new ProcessorsWhitelistExtension());     // module: ingest-common
extensions.add(new SpatialPainlessExtension());         // module: spatial
extensions.add(new WildcardPainlessExtension());