Tests in the source tree

There are some test classes located under the main java source tree.
This is then 'fixed' in the build.gradle with this
```
// Customize source code layout
sourceSets {
main {
java {
srcDirs = ['java']
exclude 'java/com/google/zetasketch/testing/**'
}
proto {
srcDirs = ['proto']
}
}
test {
java {
srcDirs = ['java/com/google/zetasketch/testing', 'javatests']
}
}
}
```
A consequence of this very strange construct is that IntelliJ gets really confused and it no longer automatically sees the sources as sources but instead it sees one level deeper as being test sources.


Why not simply move the test classes to the place where all test classes live?