Added specs for relation read

[jodosha]

This is a new feature introduced by rom-sql 0.9.1, as result of the following discussions:

• Count method #351
• Introduce Repository#connection #354
• Introducing Hanami::Repository#query #342
• Perform raw sql in a Hanami::Repository subclass #345

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This feature allows to read entities starting from a raw query string (eg SQL).

Because it's exposed by ROM relations, the purpose of this PR is to add a unit test for it to prevent regressions.

[AlfonsoUceda]

@jodosha it's perfect, let me point out, this is the commit where @solnic added that feature.