minor #46739 [Yaml] Improve test coverage in DumperTest and ParserTes…

…t (donquixote)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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[Yaml] Improve test coverage in DumperTest and ParserTest

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | this just adds test coverage
| New feature?  | this just adds test coverage
| Deprecations? | no
| Tickets       | -->
| License       | MIT
| Doc PR        | -

Note that these tests are slightly different against 4.4 then they would be against 6.1 or 6.2.
I also tried to be compatible with PHP 7.1, e.g. I did not use null coalesce operator.

Commits
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21515b32da [Yaml] Improve test coverage in DumperTest and ParserTest