Verifies that mixed content issue is created from mixed content image.

Inspector issue: {
    issue : {
        code : MixedContentIssue
        details : {
            mixedContentIssueDetails : {
                frame : {
                    frameId : <string>
                }
                insecureURL : http://devtools.test:8000/inspector-protocol/resources/img.png
                mainResourceURL : https://devtools.test:8443/inspector-protocol/resources/mixed-content-img.html
                request : {
                    requestId : <string>
                    url : http://devtools.test:8000/inspector-protocol/resources/img.png
                }
                resolutionStatus : MixedContentAutomaticallyUpgraded
                resourceType : Image
            }
        }
    }
}