make tribe-node-license validation assume snapshot context (elastic/x-pack-elasticsearch#2589)

Release tests were introduced that sets the `build.snapshot`
system property to `false` to mimic release builds. This invalidates
the hardcoded license signatures that were signed against the
integration test pub/priv keys. This commit modifies the
license-validation assertions to assume the test is running against
those test keys, and will be skipped/ignored when these assertions
fail (which should only occur with `build.snapshot=true`)

Original commit: elastic/x-pack-elasticsearch@871704a3af

qa/tribe-tests-with-license/src/test/java/org/elasticsearch/test/LicensingTribeIT.java

@@ -5,6 +5,7 @@
  */
 package org.elasticsearch.test;
 
+import org.elasticsearch.Build;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.transport.TransportAddress;
@@ -101,6 +102,7 @@ public class LicensingTribeIT extends ESIntegTestCase {
     }
 
     public void testLicensePropagateToTribeNode() throws Exception {
+        assumeTrue("License is only valid when tested against snapshot/test keys", Build.CURRENT.isSnapshot());
         // test that auto-generated trial license propagates to tribe
         assertBusy(() -> {
             GetLicenseResponse getLicenseResponse = new LicensingClient(tribeNode.client()).prepareGetLicense().get();