Add test for example and template to be valid frictionless datapackage

[Ludee]

Summary of the discussion

Use frictionless-py validate function in tests.

Type of change (CHANGELOG.md)

Added

• Tests for example and template to be valid frictionless datapackage (#305)

Workflow checklist

Automation

Closes #290 / Part of #296

PR-Assignee

• 🐙 Follow the workflow in CONTRIBUTING.md
• 📝 Update the CHANGELOG.md
• 📙 Update the documentation
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Reviewer

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• 🐙 Provided feedback and show sufficient appreciation for the work done

[jh-RLI]

I suggest tp focus on this PR before continuing work on the other "oem content" related PR´s

Most tests are failing as you used "type": "text", instead of "type": "string",

Please also put the option of "null" if a property is not required:

          "type": [
            "string",
            "null"
          ],