Introduce test fixtures

This also removes the powermock dependency. Although powermock is a
fine library, it currently prevents dropping Junit4. And since we only
use the Whitebox API of powermock, this simply replaced powermock's
Whitebox with our own.

smack-resolver-minidns-dox/build.gradle

@@ -4,5 +4,4 @@ DNS over XMPP (DoX) support using MiniDNS."""
 dependencies {
 	compile project(path: ':smack-resolver-minidns')
 	compile project(path: ':smack-experimental')
-	testCompile project(path: ":smack-core", configuration: "testRuntime")
 }