Verify RegistryTool can instantiate (#400)

* Verify RegistryTool can instantiate

Add a task that instantiates all command classes in RegistryTool
with runtimeClasspath.

Also make sure that runtimeClasspath is a superset of
compileClasspath.

core/src/main/java/google/registry/tools/RegistryCli.java

@@ -104,6 +104,8 @@ final class RegistryCli implements AutoCloseable, CommandRunner {
     // Create all command instances. It would be preferrable to do this in the constructor, but
     // JCommander mutates the command instances and doesn't reset them so we have to do it for every
     // run.
+    // TODO(weiminyu): extract this into a standalone static method to simplify
+    //   :core:registryToolIntegrationTest
     try {
       for (Map.Entry<String, ? extends Class<? extends Command>> entry : commands.entrySet()) {
         Command command = entry.getValue().getDeclaredConstructor().newInstance();