Header.Contains tests lack tests

[davecheney]

Header Contains conditions that result in a regex matcher are not tested. Tests at the internal/contour and internal/featuretest level should be added.

[stevesloka]

@davecheney there isn't any logic in internal/contour for regex other than sorting them. There are tests in internal/envoy where the regex is applied. What were your thoughts for tests inside internal/contour?