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.
The previously used approach of
project(':smack-core').sourceSets.test.runtimeClasspath
caused the 'eclipse' target to produce duplicate classpath entries in
.classpath when run with Gradle >= 2.6. It also relied on Gradle
internals.
Instead we now use
project(path: ":smack-core", configuration: "testRuntime")
project(path: ":smack-core", configuration: "archives")
to be able to use test classes from other subprojects (usually
smack-core) in e.g. smack-extensions. The 'archives' configuration
includes the test jar.
See also https://discuss.gradle.org/t/11784
Thanks to Lari Hotari for helping with this issue.
