Refactor tests with an assertion library (AssertJ or equivalent)

[clementguillot]

Description

Since I'm using kotlin-test, I can't say that I'm 100% convinced. In Spring Boot ecosystem, spring-boot-starter-test includes assertj-code which is a very nice library for Java.

We can use AssertJ in Kotlin but we would not use 100% of Kotlin syntax (and its syntax sugars). Sounds like Atrium has solid arguments:

• Still actively maintained
• Kotlin-written
• Nice fluent (AssertJ-like) API
• Lack of Coroutine support expect..toThrow does not support suspend functions robstoll/atrium#363, but there is a workaround by using runBlocking in the expect block