Fix dry-run output and add tests

[nebulon42]

Currently the dry-run status output only shows metadata files. This PR fixes that and also adds tests for the DryRunTemplateManager class.
Fixes #15730.

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[nebulon42]

Not sure what the test failure is about. Locally, I did not have any failures.

[wing328]

restarted and all tests passed

[wing328]

cc @OpenAPITools/generator-core-team