Make LegacyMoxyConsumerTest a @SpringBootTest

Make LegacyMoxyConsumerTest a @SpringBootTest

- replace AAISetup with @SpringBootTest in LegacyMoxyConsumerTest
- introduce WebTestClient to more elegantly declare requests
- do not statically invoke AAIGraph.getInstance in some tests [1]

[1] If those tests are run individually, launching the graph will fail
since the spring context is not available then.
It works in the pipeline, since other tests run before that launch the context.

Issue-ID: AAI-3887
Change-Id: I075c1f94e0a519d5cfa760cc07e446a26549c699
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>