Functional test setup with kbn-test package (#18568)

Restructure testing with kbn-test package

 - Run with multiple configs, move cli options to config
 - Package-ify kbn-test
 - Eventually we'll have functional_test_runner live in a package
of its own, and then this kbn-test will use that as a dependency,
probably still as a devDependency.
 - Implement functional_tests_server
 - Collapse single and multiple config apis into one command

Use kbn-es

Replace es_test_cluster + es_test_config with kbn/test utils

Implement new createEsTestCluster

Improve scripts, jsdocs, cli top-level tools

Lift error handling to the top level

scripts/functional_tests.js

@@ -0,0 +1,5 @@
+require('../src/babel-register');
+require('../packages/kbn-test').runTestsCli([
+  require.resolve('../test/functional/config.js'),
+  require.resolve('../test/api_integration/config.js'),
+]);