domain and service layer tests should not use code from web.rest layer

##### **Overview of the issue** #10686 moved Entity and DTO tests away from `web.rest` layer but related `TestUtil` remained in `web.rest` layer. https://github.com/jhipster/generator-jhipster/blob/0fe9a4023ac3c179f0d2de5539ba0ce28b186126/generators/java/generators/domain/templates/src/test/java/_package_/_entityPackage_/domain/_persistClass_Test.java.ejs#L34 https://github.com/jhipster/generator-jhipster/blob/0fe9a4023ac3c179f0d2de5539ba0ce28b186126/generators/spring-boot/templates/src/test/java/_package_/_entityPackage_/service/dto/_dtoClass_Test.java.ejs#L23 Additional note: #10686 moved entities domain/service layer tests from web.rest layer to domain/service layer but missed moving `User` entity tests from `web.rest` layer to `doamin` layer: https://github.com/jhipster/generator-jhipster/blob/0fe9a4023ac3c179f0d2de5539ba0ce28b186126/generators/spring-boot/templates/src/test/java/_package_/_entityPackage_/web/rest/UserResourceIT.java.ejs#L894 ##### **Motivation for or Use Case** `domain` and `service` layers should not depend on `web.rest` layer ##### **Reproduce the error** ##### **Related issues** ##### **Suggest a Fix** 1. wrong layer usage possible solutions: * possible solution 1: move `TestUtil` from web.rest to tests root * possible solution 2: create `DomainTestUtil`, `ServiceTestUtil` and rename `TestUtil` to `RestTestUtil` 2. `User` entity tests in wrong package solution: extract `User` entity tests from `web.rest` layer to `domain` layer as this was done for entities in #10686 ##### **JHipster Version(s)** Current main branch, version 8.7.1 ##### **JHipster configuration** To provide all information we need, you should run `jhipster info` in the project root folder (or workspaces root for microservices), and copy/paste the result here. `jhipster info` removes sentitive information like rememberKey, jwtSecretKey. Double check if there is any other sensitive info. As alternative you can add a JDL wrapped in below structure JDL definitions

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