Add tests for hasMany relation source code

[b-admike]

Description / Acceptance Criteria:

Add appropriate tests for the following files introduced in #1342.

• constraint-utils.ts https://github.com/strongloop/loopback-next/blob/066dfa3fc8a24f0e7ce7756673ba8d60af7fae62/packages/repository/src/repositories/constraint-utils.ts
• relation-factory.ts https://github.com/strongloop/loopback-next/blob/066dfa3fc8a24f0e7ce7756673ba8d60af7fae62/packages/repository/src/repositories/relation.factory.ts
• relation-repository https://github.com/strongloop/loopback-next/blob/066dfa3fc8a24f0e7ce7756673ba8d60af7fae62/packages/repository/src/repositories/relation.repository.ts

[dhmlau]

@b-admike , is this working in progress now? if not, is it intended for DP3?

[b-admike]

@dhmlau yeah I have started working on it with #1383 (I think it's good to have in DP3).

[b-admike]

Done in #1383.