Add support for `.exactly` to the `have_relationships(...)` matcher

[lukemelia]

What is the current behavior?

Currently, the library supports have_attributes(attr1, attr2, etc).exactly to verify that the specified attributes are the complete set of attributes in the node. However have_relationships(rel1, rel2).exactly is not supported.

What is the new behavior?

have_relationships(rel1, rel2).exactly is supported and will fail if any other relationships are present besides the specified ones.

Checklist

Please make sure the following requirements are complete:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [ ] All automated checks pass (CI/CD)

[stas]

Thank you Luke! :bow: