Merge branch '276-introduction-integration-tests' into 'master'

Introduction Integration Tests

This introduces these integration tests for the introduction client:
* normal session where both introducees accept
* normal session where the first introducee declines
* normal session where the second introducee declines
* one session where a contact is introduced to herself
* one session where two identities of the same contact are introduced to each other

I managed to get rid of the non-determinism missing validator problem by properly injecting the eager singletons.

Attention: This is based on !143 which addresses a corner case that is already tested here.

See merge request !139