rapid-cdn: Fix test instability

In case of unavailable backend 502 and 503 are the possible codes, so relax
the assertion.
133 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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passed SlapOS.Theia.ResilienceTest-Master-ERP5

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed ERP5.UnitTest-Zope4Py3-Py2

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passed SlapOS.Eggs.UnitTest-Master.Python2

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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.Python3

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.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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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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