test: improve test assertions

[anyulled]

• According to the AssertJ library, the assertThat() method must be followed by an assertion invocation, otherwise what is intended to be a test doesn't actually verify anything.
• Replacing isEqualTo() with hasToString() assertion increases readability of the code for verifying toString() methods.
• replacing isNotEqualTo(0) with isNotZero() to improve readability.
• Replace assertThat(owner6.getPets().size()).isEqualTo(found + 1) with assertThat(owner6.getPets()).hasSize(found + 1); for better code readability.
• Replace JUnit's assertEqual() assertion with AssertJ assertThat().isEqualTo() for improved readability and consistent use within the project.

solves #1474

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[anyulled]

@zyberzebra Who else needs to approve this pull request for merging it?

[zyberzebra]

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