Add integration tests to verify CCS output (elastic#40038)

We recently introduced the option to minimize network roundtrips when
executing cross-cluster search requests. All the changes made around
that are separately unit tested, and there are some yaml tests that
exercise the new code-path which involves multiple coordination steps.
This commit adds new integration tests that compare the output given by
CCS when running the same queries using the two different execution
modes available.

Relates to elastic#32125

qa/multi-cluster-search/build.gradle

@@ -21,6 +21,10 @@ import org.elasticsearch.gradle.test.RestIntegTestTask
 
 apply plugin: 'elasticsearch.standalone-test'
 
+dependencies {
+  testCompile "org.elasticsearch.client:elasticsearch-rest-high-level-client:${version}"
+}
+
 task remoteClusterTest(type: RestIntegTestTask) {
   mustRunAfter(precommit)
 }
@@ -53,6 +57,6 @@ task integTest {
   dependsOn = [mixedClusterTest]
 }
 
-unitTest.enabled = false // no unit tests for multi-cluster-search, only the rest integration test
+unitTest.enabled = false // no unit tests for multi-cluster-search, only integration tests
 
 check.dependsOn(integTest)