[TEST] Added TESTING.asciidoc with some info on testing

The goal of TESTING.asciidoc is not to repeat what's already in the elasticsearch core test cheatsheet, but only add what is different in shield.

Original commit: elastic/x-pack-elasticsearch@51ad3894d2

TESTING.asciidoc

@@ -0,0 +1,40 @@
+[[Testing Framework Cheatsheet]]
+= Testing
+
+[partintro]
+
+Elasticsearch and Shield use jUnit for testing, they also use randomness
+in the tests, that can be set using a seed, please refer to the
+Elasticsearch TESTING.asciidoc cheatsheet to know all about it.
+
+Tests are executed with network transport and unicast discovery, as this is
+the configuration that's secured by shield.
+
+---------------------------------------------------------------------------
+
+== Testing the REST layer
+
+The available integration tests are specific for Shield functionalities
+and make use of the java API to communicate with the elasticsearch nodes,
+using the internal binary transport (port 9300 by default).
+Shield is also tested using the REST tests provided by Elasticsearch core,
+just by running those same tests against a cluster with Shield installed.
+
+The REST tests are not run automatically when executing the maven test
+command as they require some additional configuration. In order to run them
+you need to checkout the Elasticsearch core project and configure its
+location through system properties. Also, a blacklist is needed as some of
+the tests are known to fail against shield due to different behaviours
+introduced by the security plugin.
+
+---------------------------------------------------------------------------
+mvn clean test -Des.logger.level=TRACE -Dtests.class=*.ShieldRestTests
+-Dtests.rest=true
+-Dtests.rest.spec=/path/to/elasticsearch/rest-api-spec/api
+-Dtests.rest.suite=/path/to/elasticsearch/rest-api-spec/test
+-Dtests.rest.blacklist="indices.get/10_basic/*allow_no_indices*,cat.count/10_basic/Test cat count output,cat.aliases/10_basic/Empty cluster,indices.segments/10_basic/segments test,indices.clear_cache/10_basic/clear_cache test,indices.status/10_basic/Indices status test,cat.indices/10_basic/Test cat indices output,cat.recovery/10_basic/Test cat recovery output,cat.shards/10_basic/Test cat shards output"
+---------------------------------------------------------------------------
+
+`ShieldRestTests` is the executable test class that runs all the
+yaml suites available within the `rest-api-spec` folder.
+