test_json_schema: Always shall full diff for JSON schema problems

Without full diff it's often impossible to understand the problem of JSON
schema introduced by the developer.
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Status Job ID Name Coverage
  External
failed Cloudooo.UnitTest-Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.Eggs.UnitTest-Master.Python3Next

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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failed SlapOS.Theia.ResilienceTest-Master-ERP5

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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