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-debug-slf4j/build.gradle

@@ -6,5 +6,4 @@ Connect your favourite slf4j backend of choice to get output inside of it"""
 dependencies {
     compile project(':smack-core')
     compile 'org.slf4j:slf4j-api:[1.7,1.8)'
-    testCompile project(path: ":smack-core", configuration: "testRuntime")
 }