Make test source location configurable via sourceSet

Suggestion for fixing https://github.com/JCAndKSolutions/android-unit-test/issues/32
Gradle code to use this would look like:
```
androidUnitTest {
    downloadDependenciesSources false
    downloadTestDependenciesSources false
    sourceSets {
    	unitTest {
    		java.srcDirs = ['../testproject/src']
    	}
    }
}
```
The fixed paths currently used would still be applied no corresponding source set is provided.