Don't dump all flavors test class files into same directory

Dumping all compiled test class files and resources into the same directory named "test-classes" was making the plugin to execute the android flavor test for all android flavors.

Example of the issue:

```groovy
android {
    productFlavors { prod{ }; beta{ } }
    androidTest {
        java.srcDirs = ['commonTestDir']
    }
    androidTestProd {
        java.srcDirs = ['someCustomTestFolder']
    }
}
```

For the above config the test configured for the flavor `prod` is getting executed for `beta`. These two flavors should share test classes that are configured under `androidTest` not the classes that are configured for flavor specific.