Fix 'test' dependencies

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.

build.gradle

@@ -274,7 +274,7 @@ subprojects {
 		classifier = 'javadoc'
 		from javadoc.destinationDir
 	}
-	task testJar(type: Jar) {
+	task testJar(type: Jar, dependsOn: testClasses) {
 		classifier = 'tests'
 		from sourceSets.test.output
 	}
@@ -293,6 +293,10 @@ subprojects {
 		}
 	}
 
+	configurations {
+		archivesOutput.extendsFrom (testCompile)
+	}
+
 	artifacts {
 		archives sourcesJar
 		archives javadocJar