Increase code-coverage

[iakunin]

In the scope of this issue multiple pdd-puzzles should be added to decompose the whole scope of testing.

There are also useful resources concerning testing best-practices:

• https://www.freecodecamp.org/news/unit-testing-services-endpoints-and-repositories-in-spring-boot-4b7d9dc2b772/
• https://www.baeldung.com/integration-testing-in-spring
• https://www.baeldung.com/spring-boot-testing
• https://dzone.com/articles/unit-and-integration-tests-in-spring-boot-2
• https://phauer.com/2019/modern-best-practices-testing-java/
• https://phauer.com/2019/focus-integration-tests-mock-based-tests/
• https://martinfowler.com/articles/practical-test-pyramid.html
• https://tanzu.vmware.com/content/blog/what-is-a-unit-test-the-answer-might-surprise-you

[0pdd]

@iakunin 11 puzzles #54, #55, #56, #57, #58, #59, #60, #61, #62, #63, #64 are still not solved.

[0pdd]

@iakunin 10 puzzles #54, #55, #56, #57, #58, #59, #60, #61, #62, #64 are still not solved; solved: #63.

[0pdd]

@iakunin 9 puzzles #54, #55, #56, #57, #59, #60, #61, #62, #64 are still not solved; solved: #58, #63.

[0pdd]

@iakunin 8 puzzles #54, #55, #56, #59, #60, #61, #62, #64 are still not solved; solved: #57, #58, #63.

[0pdd]

@iakunin 7 puzzles #54, #55, #56, #59, #60, #61, #62 are still not solved; solved: #57, #58, #63, #64.

[0pdd]

@iakunin 6 puzzles #54, #55, #56, #59, #60, #61 are still not solved; solved: #57, #58, #62, #63, #64.

[0pdd]

@iakunin 5 puzzles #54, #55, #56, #59, #60 are still not solved; solved: #57, #58, #61, #62, #63, #64.

[0pdd]

@iakunin the puzzle #54 is still not solved; solved: #55, #56, #57, #58, #59, #60, #61, #62, #63, #64.