Consider backend TCK and test utils as test code during the Sonar analysis

Currently the Sonar analysis considers the backend TCK and test utils as "main" code because this code is in the src/main directory. But we only put the code there to follow the preferred way to create JARs containing test code, which allows transitive dependencies.

As a result, about 400 issues were created in Sonar that do not make sense in test code, such as "use constants for repeated string values".

We should tune the Sonar analysis to fix this.