Add initializer tests and move classpath files

The initializer tests verify that every non-optional initializer, this
includes Providers, is loadable.

Creating files under META-INF is not considered best practice. Smack's
configuration and provider files reside now in classpath directory
qualified by Smack's package namespace.

extensions/src/test/java/org/jivesoftware/smackx/ExtensionsProviderInitializerTest.java

@@ -0,0 +1,16 @@
+package org.jivesoftware.smackx;
+
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+public class ExtensionsProviderInitializerTest {
+
+    @Test
+    public void testExtensionProviderInitializer() {
+        ExtensionsProviderInitializer ei = new ExtensionsProviderInitializer();
+        ei.initialize();
+        assertTrue(ei.getExceptions().size() == 0);
+    }
+
+}

extensions/src/test/java/org/jivesoftware/smackx/ExtensionsStartupClassesTest.java

@@ -0,0 +1,16 @@
+package org.jivesoftware.smackx;
+
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+public class ExtensionsStartupClassesTest {
+
+    @Test
+    public void testExtensiosnStartupClasses() {
+        ExtensionsStartupClasses esc = new ExtensionsStartupClasses();
+        esc.initialize();
+        assertTrue(esc.getExceptions().size() == 0);
+    }
+
+}