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-jingle-old/build.gradle

@@ -7,5 +7,4 @@ dependencies {
 	compile project(':smack-extensions')
 	compile 'javax.media:jmf:2.1.1e'
 	compile files('libs/jspeex-0.9.7-jfcom.jar', 'libs/jstun.jar', 'libs/Speex.jar')
-	testCompile project(path: ":smack-core", configuration: "testRuntime")
 }