Use tests.asserts flag to allow disabling assertions

Randomized runner uses a flag, tests.asserts, which we have previously
not used, but is used in lucene for disabling assertions. This change
modifies the gradle configuration to look for this flag and pass through
to the test runner to determine whether -ea and -esa are added to the
java commandline for tests.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -509,11 +509,9 @@ class BuildPlugin implements Plugin<Project> {
                 }
             }
 
-            // System assertions (-esa) are disabled for now because of what looks like a
-            // JDK bug triggered by Groovy on JDK7. We should look at re-enabling system
-            // assertions when we upgrade to a new version of Groovy (currently 2.4.4) or
-            // require JDK8. See https://issues.apache.org/jira/browse/GROOVY-7528.
-            enableSystemAssertions false
+            boolean assertionsEnabled = Boolean.parseBoolean(System.getProperty('tests.asserts', 'true'))
+            enableSystemAssertions assertionsEnabled
+            enableAssertions assertionsEnabled
 
             testLogging {
                 showNumFailuresAtEnd 25

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -72,12 +72,10 @@ class ClusterConfiguration {
     boolean useMinimumMasterNodes = true
 
     @Input
-    String jvmArgs = "-ea" +
-        " " + "-Xms" + System.getProperty('tests.heap.size', '512m') +
+    String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
         " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
         " " + System.getProperty('tests.jvm.argline', '')
 
-
     /**
      * A closure to call which returns the unicast host to connect to for cluster formation.
      *

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/NodeInfo.groovy

@@ -151,6 +151,9 @@ class NodeInfo {
         args.addAll("-E", "node.portsfile=true")
         String collectedSystemProperties = config.systemProperties.collect { key, value -> "-D${key}=${value}" }.join(" ")
         String esJavaOpts = config.jvmArgs.isEmpty() ? collectedSystemProperties : collectedSystemProperties + " " + config.jvmArgs
+        if (Boolean.parseBoolean(System.getProperty('tests.asserts', 'true'))) {
+            esJavaOpts += " -ea -esa"
+        }
         env.put('ES_JAVA_OPTS', esJavaOpts)
         for (Map.Entry<String, String> property : System.properties.entrySet()) {
             if (property.key.startsWith('tests.es.')) {