Add support for tests.jvm.argline in testclusters (#46540)

We have CI jobs that make use of this for testing alternative garbadge
collectors.

buildSrc/src/main/java/org/elasticsearch/gradle/testclusters/ElasticsearchNode.java

@@ -599,8 +599,11 @@ public class ElasticsearchNode implements TestClusterConfiguration {
                 })
                 .collect(Collectors.joining(" "));
         }
-        defaultEnv.put("ES_JAVA_OPTS", "-Xms512m -Xmx512m -ea -esa" +
-            systemPropertiesString + jvmArgsString
+        defaultEnv.put("ES_JAVA_OPTS", "-Xms512m -Xmx512m -ea -esa " +
+            systemPropertiesString + " " +
+            jvmArgsString + " " +
+            // Support  passing in additional JVM arguments
+            System.getProperty("tests.jvm.argline", "")
         );
         defaultEnv.put("ES_TMPDIR", tmpDir.toString());
         // Windows requires this as it defaults to `c:\windows` despite ES_TMPDIR