Add E2E tests for "kn service export"

[rhuss]

kn export needs some E2E testing love. For the initial implementation, see #669

/assign itsmurugappan

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In response to this:

kn export needs some E2E testing love. For the initial implementation, see #669

/assign itsmurugappan

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[itsmurugappan]

/assign @itsmurugappan

[itsmurugappan]

This is completed via #739

[rhuss]

thanks !

/close

[knative-prow-robot]

@rhuss: Closing this issue.

Details

In response to this:

thanks !

/close

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