Consolidate test validation

[stuebingerb]

Currently, validations in test functions are implemented in different ways:

• Using kluent, e.g.

  invoking {
      deserialize(schema.executeBlocking("{b{id, name, age}}"))
  } shouldThrow GraphQLError::class withMessage "Property id on B does not exist"
  

• Using custom implementations, e.g.

  expect<IllegalAccessException>("ILLEGAL ACCESS") {
      schema.executeBlocking("{black_mamba {item}}", context = context { +"LAKERS" }).also(::println)
  }
  

• Using hamcrest, e.g.

  assertThat(scenarioType["author"], nullValue())
  

This makes it unnecessarily complex and hard to maintain and should be consolidated, ideally towards a single, kotlin-native test library.

Ideas for alternatives:

• https://kotlinlang.org/docs/power-assert.html
• https://kotest.io/docs/assertions/assertions.html
• https://github.com/robstoll/atrium
• https://github.com/willowtreeapps/assertk