changed some PHPUnit assertions to more specific ones
minor #10358 PHPUnit more specific assertions (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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PHPUnit more specific assertions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10357
| License       | MIT
| Doc PR        | n/a

Commits
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4927d0c changed some PHPUnit assertions to more specific ones