Validate stdin, stdinOnce, and tty cannot be set in conformance tests

[dgerd]

In what area(s)?

/area API

/area autoscale
/area build
/area monitoring
/area networking
/area test-and-release

Describe the feature

We currently validate this in unit tests, but we should add conformance tests to https://github.com/knative/serving/blob/master/test/conformance/container_test.go as not having terminal is part of the runtime contract process section

[mattmoor]

/assign @tzununbekov

[knative-prow-robot]

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Details

In response to this:

/assign @tzununbekov

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