test: hard code esApiVersion to master (#13198)

Until elasticsearch.js actually has support for 6.x/6.0 branches, this
needs to just rely on the master apiVersion.

Also, we shouldn't change this esApiVersion in tests unless we are also
changing the value in our elasticsearch plugin proper, otherwise our
tests can accidentally use a different API version than reality.

src/core_plugins/tests_bundle/tests_entry_template.js

@@ -1,5 +1,3 @@
-import { esTestConfig } from '../../test_utils/es';
-
 export default function ({ env, bundle }) {
 
   const pluginSlug = env.pluginInfo.sort()
@@ -28,7 +26,7 @@ window.__KBN__ = {
   vars: {
     kbnIndex: '.kibana',
     esShardTimeout: 1500,
-    esApiVersion: ${JSON.stringify(esTestConfig.getBranch())},
+    esApiVersion: 'master',
     esRequestTimeout: '300000',
     tilemapsConfig: {
       deprecated: {