Update guide-testing.asciidoc

[dhelmke]

Like Jörg described here: #182

There are different aspects in this testing guide:
• Example for using Mockito: Needs to be updated to JUnit5. An example can be found here: https://github.com/devonfw/my-thai-star/blob/1825456aa52711c1a4103d4428e0728d1e0f0bd8/java/mtsj/core/src/test/java/com/devonfw/application/mtsj/clustermanagement/logic/impl/ClustermanagementTest.java - However, I think that the demonstration of Mockito does not have to be related to our example app (MTS) in any way. You can also get instructions/example from the web - e.g.: https://dzone.com/articles/spring-boot-2-with-junit-5-and-mockito-2-for-unit
• Example for subsystem-tests: Here the entire section is titled TODO so the current state is a mess already. We could simply give our REST testing recepie here: https://github.com/devonfw/my-thai-star/blob/ac7f2f51d109d8e9d9fdc1aade7f662f58f87024/java/mtsj/core/src/test/java/com/devonfw/application/mtsj/general/common/base/AbstractRestServiceTest.java - However, also I am thinking that this testing guide should stay rather highlevel. We should more go into a direction that when we explain features like REST or JPA that we should include a section on testing. The testing guide might link to all such places. Or is it better to have everything in that testing guide?

Link to the file: https://github.com/devonfw/devon4j/blob/develop/documentation/guide-testing.asciidoc