Index executors: Use directExecutor for negative thread values

This will make it easier to test index-related logic in tests. We still
use 0 as a default way of setting the JVM threads.

Change-Id: If186a23ee4e79c49306bf9f286a24841d99e6531

java/com/google/gerrit/acceptance/AbstractDaemonTest.java

@@ -397,6 +397,8 @@ public abstract class AbstractDaemonTest {
       baseConfig.setString("sshd", null, "listenAddress", "off");
     }
 
+    baseConfig.setInt("index", null, "batchThreads", -1);
+
     baseConfig.setInt("receive", null, "changeUpdateThreads", 4);
     Module module = createModule();
     if (classDesc.equals(methodDesc) && !classDesc.sandboxed() && !methodDesc.sandboxed()) {